Liselotte Neumann, of Sweden, is in second place with Switzerland's Evelyn Orley, Corinne Dibnah, the former champion from Australia, and the New Zealander Janice Arnold a further stroke behind.
Sheehan, who is attempting to become the first player to win both the US and British Open titles in the same year, only received the call to play here last Monday when Marta Figueras- Dotti, of Spain, withdrew.
'I had wanted to play at the start of the year but I was only the fifth alternate. I would never pay the dollars 10,000 ( pounds 6,000) fine to play,' Sheehan said in reference to the fine which has been imposed on Scotland's Pamela Wright for competing here without an official release from the US tour. 'This is a ridiculous fine and I think that our Commissioner could have made an exception to the rule,' Sheehan added.
'This tournament could have a big impact on the European Tour and all the help we can give will be good for women's golf.'
Sheehan started her round from the 10th tee and admitted to being 'a little rusty' after taking three weeks off. This showed at the outset when her opening five holes produced mixed results, two birdies, a double-bogey and a bogey taking her to the turn in 38.
Two birdies in a row from the first were followed by fours at the long fourth and sixth holes before Sheehan completed a stunning homeward run of 30 with putts of 12 and 15 feet for birdies at her last two holes.
Sheehan played alongside Neumann and Laura Davies in the worst of the wet morning weather. Neumann, who is having daily treatment from a chiropractor for a back injury, also played the final nine holes in 30 for a 69 which she rounded off with a 20- foot putt for a birdie at the ninth.
Davies hit her opening drive into a bush from where she scored a double-bogey seven. 'It was a revolting start. I was expecting a birdie but I didn't get worked up. There is still a long way to go,' Davies, who is five strokes off the lead, said.
Great Britain and Ireland took a three-shot lead in the opening round of the Women's World Team Championship in Vancouver with a one-over-par 145.
WEETABIX WOMEN'S BRITISH OPEN (Duke's Course, Woburn) Leading first-round scores (GB or Ire unless stated): 68 P Sheehan (US). 69 L Neumann (Swe). 70 E Orley (Swit), C Dibnah (Aus), J Arnold (NZ). 71 C Figg Courier (US), V Michaud (Fr), F Descampe (Bel), M De Lorenzi (Fr). 72 A Nicholas, M Burstrom (Swe), T Abitbol (Sp), P Rizzo (US), K Parker (US), K Orum (Den). 73 M Hageman (Neth), C Nilsmark (Swe), H Wadsworth, L Davies, S Rule (US), F Dassu (It), K Cornelius (US), D Reid, K Douglas, T Johnson, P Wright, E Farquharson. 74 C Savy (Aus), D Mochrie (US), H Alfredsson (Swe), J Posener (Swe), K Marshall, L Mills (US), J Sevil (Aus), C Hjalmarsson (Swe), L Hackney, B New, A Shapcott, L Fairclough.
Woosnam in swing, page 35
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