THE British Lions embark upon what coach Ian McGeechan has identified as the toughest seven days of rugby that any British or Irish player has ever experienced today with the encounter against Northern Transvaal in Pretoria.

They enter the clash against the Blue Bulls at Loftus Versfeld with confidence on a high after four straight wins and the knowledge that, with each victory, they have earned themselves the respect of the South African public - a respect that simply was not there when they arrived in the Republic three weeks ago. After today's game against Northern Transvaal, the Lions play Gauteng Lions (Transvaal) in Johannesburg on Wednesday and then take on Currie Cup champions Natal in Durban on Saturday.

Thereafter, the schedule does not become much easier, with the Emerging Springboks to be taken care of midweek before the first Test in Cape Town in a fortnight's time. Such is the intensity of the schedule that the Emerging 'Boks are viewed as the only softish option on the card before the Test series gets under way.

Northern Transvaal are the first of South Africa's Super 12 outfits to be encountered on this tour. They will be without five of their Springboks, who are in camp with the national squad in preparation for the warm-up Test against Tonga in Cape Town on Tuesday.

Inspirational scrum-half Joost van der Westhuizen will be the biggest miss for Northern Transvaal today, but Andre Snyman, Krynauw Otto, Ruben Kruger, and Marius Hurter are also unavailable due to their involvement with the Bokke in Cape Town. Still, though, the Blue Bulls, who held Super 12 champions Auckland to a 40-40 draw, are able to call upon the services of four Springboks of recent vintage - Adriaan Richter, Schutte Bekker, Danie van Schalkwyk, and Henry Tromp.

The Namibian-born hooker Tromp is a particularly interesting character. The 30-year-old made his Springbok debut against New Zealand in Durban last year with a storm-sized cloud hanging over him. Two years previously, he had been sentenced to two years in prison for the manslaughter of a black farmworker.

Tromp was implicated in having tied the victim to a tree and having beaten him with a fan-belt. The defence claimed that the dead man had stolen some money. Tromp was released after serving six months and within the year had been selected to represent his country.

The Lions trained in the magnificent Loftus Versfeld stadium yesterday, a light session encompassing a team run, a few lineouts, and restart drills. The England lock, Simon Shaw, who is paired alongside tour captain Martin Johnson - playing only his second game - was an early casualty as he landed badly on an ankle and sat out most of the session. However, McGeechan said that Shaw is fit and will be in the starting line-up today.

McGeechan declared: ''Once again, today, we are asking the players to increase the inten-sity of their rugby. We meet the first of the Super 12 opposition.

''We have admired Super 12 rugby from afar and we know that we will have to reach new peaks if we are to do well.

''There are staging posts all the way through a tour like this and on Saturday we enter a particularly intensive period. We know that if we are to acquit ourselves well in the Tests then we have to maintain the progress that we have been making since arriving here.

''Basically, we are looking at the hardest period of rugby that any of us have ever been involved in,'' said McGeechan. He has coached three consecutive Lions' sides and is, himself, a double Lion as a player. That kind of gilt-edged background places in stark context the huge challenge that these Lions now face.

A challenge of a more speci-fic kind also faces the Lions today. The pack that played Mpumalanga earned plaudits on Wednesday for the startling improvement in the quality of their scrummaging. That progress will have to be maintained by today's eight if the Lions are not to find themselves back at square one.

There is now little doubt that Johnson is being wrapped in cotton-wool in order to keep him hale and hearty for the Test matches. He played against Western Province last weekend and, after three weeks, plays again for only the second time today.

He is reckoned to be suffering from a long-term groin injury and may, too, have a dodgy shoulder. All of the medi- cal expertise that the Lions can muster will be brought to bear so that he can lead his Lions in the three-Test series.

Yesterday, Johnson maintained again that his low-key involvement was part of a pre-determined strategy, but he said, too, that his feeling was that, as the tour has progressed, South Africa is beginning to take the '97 Lions more seriously.

British Lions - T Stimpson; J Bentley (Newcastle and England), A Tait (New-castle and Scotland), J Guscott (Bath and England), T Underwood (Newcastle and England); G Townsend (Northampton and Scotland), R Howley (Cardiff and Wales); G Rowntree (Leicester and England), M Regan (Bristol and England), J Leonard (Harlequins and England), M Johnson, captain, (Leicester and England), S Shaw (Bristol and England), L Dallaglio (Wasps and England), S Quinnell (Richmond and Wales), E Miller (Leicester and Ireland). Substitutes - S Gibbs (Swansea and Wales), M Catt (Bath and England), A Healey (Leicester and England), B Williams (Richmond and Wales), D Young (Cardiff and Wales), T Rodber (Northampton and England).

Northern Transvaal - G Bouwer; C Steyn, J Schutte, D van Schalkwyk, W Lourens; R de Marigny, C Breytenback; A Richter, captain; L Campher, H Tromp, P Boer, D Grobelaar, D Badenhorst, N van der Walt, S Bekker.