League Two strugglers Barnet produced a superb second-half comeback to send Northampton crashing out of the FA Cup.
The Cobblers took the lead just before the break when former Barnet striker Scott McGleish headed powerfully home.
Adam Birchall wriggled through the defence to score his first goal for Barnet on 48 minutes and soon after Dean Sinclair's header made it 2-1.
Ian Hendon slammed home the third from the penalty spot on 66 minutes before substitute Magno Vieira added a fourth.
Barnet: Harrison, Hendon, Yakubu, King, Gross (Devera 74), Cogan, Birchall (Vieira 77), Bailey, Sinclair, Puncheon, Hatch.
Subs Not Used: Flitney, Graham, Warhurst.
Booked: Hendon, Hatch.
Goals: Birchall 49, Sinclair 53, Hendon 67 pen, Vieira 87.
Northampton: Bunn, Holt, Doig, Dyche, Chambers, Burnell, Taylor, Hunt (Watt 68), Jess (Wright 58), Quinn (Kirk 58), McGleish.
Subs Not Used: Bojic, Gilligan.
Booked: Taylor.
Goals: McGleish 45.
Att: 2,786.
Ref: K Stroud (Dorset).