Northants captain Alex Wakely scored a century as their match at Derbyshire remained in the balance after day two.
Resuming on 74-3, trailing by 186 runs, Wakely built a 120-run sixth-wicket partnership with Steven Crook (60) before being caught at slip for 106.
Derbyshire's Tony Palladino and Matt Critchley took four wickets each as the visitors made 289 to lead by 29 runs.
The hosts reached 147-4 in their second innings by close, a lead of 118 runs, with Wayne Madsen 52 not out.
Seamer Ben Sanderson took the wickets of Derbyshire skipper Billy Godleman and nightwatchman Dan Wheeldon to add to his five-wicket haul earlier in the match, as Northants ended the day on top.