Sascha Kindred delivered a devastating display to defend his 200m individual medley SM6 title and win Britain's first swimming gold in Athens.
The Manchester-based athlete won in a new Paralympic record time of two minutes, 51.39, 12m clear of the field.
In the women's event, Natalie Jones held off team-mate Nyree Lewis to claim a surprise gold in a world record time.
The 19-year-old took the lead in the final 50m freestyle to come home first in 3:18.68 while Lewis took silver.
"It was hard work," three-time Paralympic gold medallist Kindred told BBC Sport.
"I went into the race ranked as world number one so the pressure was on me but I'm happy with the result."
Jones was lost for words after winning her first Paralympic medal at her second Games.
"I can't believe it," said the Colchester-based swimmer.
"I'm so stunned, it's just unbelievable."
Lewis, who goes in her strongest event the 100m backstroke on Tuesday, was satisfied with silver.
"I came fourth at the Sydney Games so this is a big improvement."
Fellow Welsh athlete Dave Roberts was also pleased with his silver in the men's SM7 200m individual medley.
He battled back over the last 50m to finish behind American Rudy Garcia in 2:47.03.
"I only took this event up six months ago so I'm delighted," said Roberts.
"I have my 100m freestyle to come so hopefully I can turn this silver into gold."
Welsh teenager Rhiannon Henry started Great Britain's medal haul in the pool with a bronze medal.
The 17-year-old Paralympic debutant recorded a time of one minute, 09.21 seconds to take third in the S13 women's 100m butterfly final.
Fran Williamson added to the tally at the Acquatics Centre two races later when she also secured a bronze in the 100m S3 freestyle final.
Giles Long missed out on his chance to complete a hat-trick of S8 100m butterfly crown.
The London-based swimmer could not match the pace of Chinese teenager Xiao Fu or Australian Ben Austin and had to settle for bronze.
Anthony Stephens picked up a fourth bronze for Britain in the men's S5 100m freestlye.