The Cobblers were thumped 3-0 by Rochdale as their six-game unbeaten run was ended in emphatic style.
Former Sixfields striker Ian Henderson gave Dale they lead after only five minutes and they added to it with second-half goals from Matty Lund and Joe Bunney.
Lund was sent off for celebrating his goal but even against 10 men, Northampton were unable to fashion any consistent attacks.
Rochdale - who went top of league two with the win - were the better side in every department, controlled the game from start to finish and perhaps should have won by a greater margin.
As in the game between the sides at Spotland, Henderson scored Dale’s first goal, collecting a knockdown from a deep free-kick in enough space to get a shot away after only five minutes.
His connection, with his less favoured left foot, was not clean but he got enough on the shot to steer it past Matt Duke’s left hand and inside the far post.
Rochdale were in control of the match for long periods of the first half.
Their well-drilled and set defence used its physical attributes to easily contain Emile Sinclair, who again looked off the pace.
And their five-man midfield allowed the home side to have possession in their own half but pressed them relentlessly in its own, forcing them to go long from the back and limit their quality possession enormously.
On the ball they were much more relaxed and confident and, as a result, their passing and interplay was much more crisp.
For what seemed like long spells the knocked the ball around almost at will and when Northampton did get the chance to attack via a set-piece they either got the delivery wrong or Dale mopped it up comfortably.
The visitors’ second goal came with a degree of inevitability and a healthy slice of controversy.
Lund dived to head home Peter Vincenti’s cross and was then booked for over-celebrating; as it was his second yellow card of the night, he duly headed off for an early bath.
Not that having a player less made any difference to Rochdale, who were in rampant mood in this game after a couple of uncertain results.
With 25 minutes to go, Northampton butchered an attack and the visitors broke on them with the end result being Joe Bunney thrashing home a third goal to rub salt into the wounds.
There was almost more as Vincenti struck the foot of the post after Henderson had beaten McSweeney for pace.
A fourth goal would have been cruel, but not undeserved.
Cobblers: Duke, McSweeney, Diamond, Doumbe, Robertson, Hackett, Ravenhill, Carter, Morris, Connell, Sinclair
Subs: Dickenson (for Diamond 46), Lopez (for Connell 63), Toney (for Robertson 79)
Not used: Snedker, Tozer, Deegan, Moyo
Rochdale: Lillis, Rafferty, O’Connell, Eastham, Done, Vincenti, Lund, Cavanagh, Allen, Henderson, Bunney
Subs: Donnelly (for Bunney 79)
Not used: Bennett, Kennedy, Porter, Tanser, Thomson, Cummins
Away fans: 181