Watford's bid to finish in the top half of the table was given a boost when manager Ray Lewington let the kids loose in a much-changed side.
The Hornets made ten changes from the line-up that lost at Bradford on Saturday but proved to be lively, sharp and too enthusiastic for a Derby side that had won three of their last four games.
Man of the match, Jamie Hand, is one of the more experienced of the relative newcomers to the Watford side, but Lewington, without any money to spend in the summer, appears to have several promising youngsters knocking at the first-team door.
Goalkeeper Richard Lee, centre-half Jerel Ifil and striker Jason Norville gave promising performances and the Watford manager admitted afterwards: "I thought they all played extremely well and the senior players, who protected them, did a first class job." Watford took the lead after 17 minutes and Lewington must wish that striker Michael Chopra, on loan from Newcastle, was a permanent part of his young squad.
Chopra, who returned to Newcastle after the game, volleyed in from eight yards when he met a ball above waist height from Stephen Glass, and fired a shot keeper Lee Grant with a well-struck volley.
It took 26 minutes before Derby produced their first attempt at goal, and then Craig Burley fired wide.
By then, Watford had had two or three good chances with the keeper denying Norville and Glass and then, after a Neal Ardely corner had been flicked on by Ifil, Chopra's shot was deflected wide.
Derby warmed to the challenge towards the end of the half and Fabrizio Ravanelli managed to escape his marker to send a Paul Boertien cross against the bar. Watford's keeper Lee then made a good save to a follow-up shot.
Watford had their chances with Glass curling a free-kick wide and Hand, breaking clear, choosing to shoot early and Grant was grateful when the ball was sent straight at him.
Just before the interval Norville set up Chopra but he fired over the bar.
Watford continued to look bright after the break but even so Derby had their moments. Ravanelli shot well wide from a good position after 61 minutes and then Burley attempted to chip goalwards, but failed to test Lee.
Watford, however, had the better moments and the greater possession.
They had to wait until the 84th minute before finally sealing their victory. Substitute Scott Fitzgerald worried the Derby defence into an error and Anthony McNamee broke down the left before cutting the ball back to meet Ardley's late run and the midfielder drove his shot home.