Rob Hulse's header gave Derby the east midlands bragging rights as Nottingham Forest's long unbeaten streak came to a shuddering end with a 1-0 defeat.
Forest arrived at Pride Park on a 19-game unbeaten league run and looking to rejoin Newcastle at the Coca-Cola Championship summit.
But a tight encounter went the way of the relegation-threatened Rams when Hulse rose to glance home Kris Commons' inswinging free-kick with 12 minutes remaining.
It always looked like one goal would be enough to win the game and Forest struggled to create any chance of note despite some late pressure as they desperately searched for the equaliser.
It was Derby's first home win since the end of November and left Forest still looking for their first-ever win at Pride Park.
There were half-chances at either end inside the opening 10 minutes.
A goalbound Robert Earnshaw volley struck a Derby defender on the six-yard line after the Rams had failed to clear their lines from a corner.
A ricochet from a corner then caused confusion inside the Forest box and Stephen Pearson hooked the loose ball wide from eight yards.
Derby would have been rewarded for a sustained period of pressure midway through the first half had it not been for the brilliance of Lee Camp in the visitors' goal.
A deep corner from Commons was met by Jake Buxton at the far post and Camp back-pedalled to tip the defender's looping header over the crossbar.
Two minutes later Camp denied his former club again. This time Shaun Barker's header from a Lee Johnson corner was pushed around a post.
Forest posed more of a threat after the break and a glorious chance to take the lead went begging after 51 minutes.
A magical run by Radoslaw Majewski saw the Pole cut infield from the left and leave a trail of Derby defenders in his wake as he reached the byline.
His poked cross along the six-yard line should have been tapped in by Paul Anderson at the far post but the midfielder was unable to adjust his feet in time and inadvertently directed the ball away from goal.
Soon afterwards Stephen Bywater made a double save to keep out Dexter Blackstock's header and Anderson's headed effort from the rebound.
At the other end, Hulse fired wide with an acrobatic volley before a timely sliding tackle by Chris Gunter midway through the second half deflected Pearson's driven shot wide.
Unlike in the first half, Derby made their pressure pay when Hulse headed Commons' cross into the roof of the net in the 78th minute.
Gunter and Jay McEveley clashed in stoppage time and, just like at the City Ground in August, a scuffle ensued involving several players from either side. Both players escaped with yellow cards.
Forest tried to get the equalising goal to preserve local pride and their unbeaten record, but their efforts were in vain and it was Derby's day on derby day in the east midlands.
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