Swindon grabbed their winner in the 90th minute to send Rushden & Diamonds out of the Carling Cup at the first hurdle.
The Robins made several changes to their line-up from Saturdays win over Bristol City.
On four minutes Lloyd Opara almost got on the end of a cross after Billy Turley failed to gather and two minutes later Diamonds skipper Stuart Gray had a golden opportunity when left unmarked from David Bell's corner, but he shot well over from ten yards out.
Grant Smith went close for Swindon with a fierce shot from 20 yards and a minute later Alex Hay almost put the Diamonds ahead, but Steve Book produced a fine save to deny the striker.
On 27 minutes a fine flowing move by the Diamonds involving Hay and Gray ended with Andrew Burgess shooting wide when he should have done better.
Smith was in the thick of the action again when he volleyed just wide following a cross by Steve Robinson. On 53 minutes Grant Howard went close for the visitors with a drive from 20 yards out.
Swindon manager Andy King made three substitutions, bringing on Sam Igoe, Sam Parkin and Sean O'Hanlon and this seemed to make a difference straight away.
On 68 minutes O'Hanlon headed just over following Igoe's corner and minutes later Parkin shot just over from ten yards out.
On 86 minutes Parkin almost broke the deadlock when he was inches wide when stretching for Igoe's cross and two minutes later Rory Fallon had the ball in the net only to be ruled offside.
Swindon grabbed the winner in the 90th minute when skipper Matt Hewlett shot home from ten yards out after Igoe's cross was not cleared and the home side were unable to put any pressure on Swindon in injury time.