Skipper Stephen Peters and Steven Crook helped Northamptonshire snatch a first-innings lead at the halfway mark against Kent.
Peters enjoyed one mid-afternoon let off, but otherwise maintained intense concentration for 297 minutes to post his first hundred of the summer (106) and help his side reach 303 in reply to Kent's first day total of 271 - a first innings lead of 32.
The visitors had to rely on Crook's 62, from 79 balls, to edge ahead after they had been 186 for seven. The tail-ender put on 43 with the injured Rob Newton for the last wicket before he was caught off Matt Coles.
That left Kent with three overs to negotiate, which they managed to be six without loss at the close.
Northamptonshire were initially made to work hard for their runs as Kent's new captain James Tredwell set intelligent fields to some naggingly accurate bowling from Charlie Shreck, Mark Davies and Calum Haggett.
Davies was first to strike, trapping his former Durham team-mate Kyle Coetzer (six) leg before when half forward then, just before lunch, David Sales, having ground 16 runs from 69 balls, edged a push drive against Shreck into the gloves of Geraint Jones.
Kent bossed the middle session, taking four wickets for 101 runs, but the situation might have been even rosier for the hosts had Jones held on to a regulation chance off Davies with Peters on 63.
Northamptonshire were hampered by the loss of Newton, their regular number five, who picked up a groin strain in the field on Wednesday and could only bat last.
As it was, Shreck had Alex Wakeley (one) caught behind when working to leg and then Andrew Hall, the former Kent all-rounder, fell leg before when shouldering arms for 12.
Jones snared another catch off the outside edge of James Middlebrook's (24) bat to give Haggett his first St Lawrence success, then Davies rushed one through the gate of David Murphy (five) to pluck out leg stump via an inside edge and make it 178 for six at tea.
Three balls after the interval David Willey (25) drove hard at a full ball from Coles to be caught behind, but Peters marched on to his 100 in a shade over four and a half hours with 11 fours. It was his 21st first-class ton since joining Northamptonshire in 2006.
The right-hander perished soon after for 106 when Davies got his second delivery with the second new ball to lift and have the visiting opener caught at short mid-wicket by 12th man Alex Blake.
Blake was in the action again when taking a sitter from Trent Copeland at mid-on to give Shreck figures of four for 80, but Newton, batting with Coetzer as a runner, helped add a vital 43 for the last wicket to ease the visitors into a slender lead and to a third batting bonus point.
Coles ended their resistance by having Crook caught at long leg, again by the substitute fielder.