Nyree Kindred set a new Paralympic record as the swimming programme got under way at the Aquatics Centre.
London 2012 is the 32-year-old's fourth Paralympics and she is looking to add to the nine medals, including two golds, she has already secured.
The Welshwoman was second in the S6 100 metres backstroke in Beijing and she booked her passage into the final in one minute 27.96 seconds, lowering by 0.22secs the mark set by China's Lu Dong in the first heat.
Kindred, who was the first Briton into the pool, said: "I couldn't ask for anything better in a heat swim."
Also impressive in qualification was Hannah Russell in the S12 400m freestyle, lowering her personal best to 4:41.25 to qualify fastest by 5.47secs.
Russell, who only recently turned 16, appears in pole position to at the very least claim a medal, if not a title, on her Paralympic debut.
Sean Fraser made it into the S8 100m butterfly final although Oliver Hynd missed out, while Robert Welbourn and Jack Bridge failed to get through in the SM10 200m individual medley.
Anthony Stephens and Andrew Mullen filled the final slots in the S5 50m freestyle although by so doing they edged out Ireland's James Scully.
Susie Rodgers reached the S7 100m backstroke decider with Stephanie Millward and Claire Cashmore both in the reckoning in the S9 100m butterfly although world record holder Natalie du Toit of South Africa was some way ahead in qualifying in her final Games.
Harriet Lee and Gemma Almond made it through in the SM10 200m individual medley in which Pascoe set the new record.