OXFORD United's automatic promotion hopes took a big dent as they were well beaten by rivals Rochdale at Spotland.
Scott Hogan's 17-minute hat-trick either side of half-time did all the damage, as the visitors crashed to their heaviest defeat for almost a year.
The visitors were left to wonder what might have been if Johnny Mullins had converted a glorious chance with the game scoreless.
But after falling behind, they had no answer to Rochdale, who consolidated their position in the top three at United's expense.
Jake Wright and Michael Raynes returned from injuries and came straight into the back four, at the expense of Scott Davies and Sam Long.
The third change to the XI from the 1-1 draw at Morecambe a week earlier saw Deane Smalley replace James Constable.
Raynes looked rusty early on and picked up an early booking, hauling down Hogan after the striker had been too quick for the centre back out wide.
It was part of an unconvincing opening 20 minutes from the visitors, who struggled to keep the ball.
Rochdale were far more coherent and looked dangerous when they came forward.
They lacked an end product, though, with Ryan Clarke's only action coming from fielding a long range shot from Ian Henderson.
United gradually warmed to their task and midway through the half should have taken the lead.
Raynes flicked on a David Hunt long throw, finding Johnny Mullins unmarked inside the six-yard box. His snap shot was blocked by goalkeeper Josh Lillis and the defender could not readjust for the rebound, which ricocheted wide.
United went on to enjoy a good spell, forcing several corners. They could not test Lillis to the same extent again and instead it was Rochdale who hit back to finish the half strongly.
There was almost a breakthrough in the 40th minute, when Mullins was writing in pain on the ground following a collision with Clarke.
Dale played on and Matt Done's low shot beat the goalkeeper, only for Wright to intercept and clear the ball behind, via the crossbar.
Mullins received treatment and returned to the fray, but United's defence stumbled a minute before the break. A cross from the left found Peter Vincenti, whose goal-bound header was turned in by Hogan a yard out.
There was a hint of offside about the goal, but there was no doubt about the striker's second, four minutes after the restart.
Collecting the ball 40 yards out, Hogan brushed off a couple of challenges and fired home at the second attempt after Clarke had blocked the initial shot.
United were ragged. A mix-up between Wroe and Williams, who was swiftly replaced by Kitson, allowed Vincenti to line up a shot which flew just wide.
The contest was effectively ended just after the hour mark. A long throw got caught under Raynes's boot and Hogan was on hand to tuck in from close range.
U's caretaker manager Mickey Lewis brought on Constable for David Connolly, but it was too little, too late.
Dale were content to allow the visitors see more of the ball, but United were rarely able to stretch their hosts.
Only an excellent reflex save from Clarke prevented Hogan from claiming a fourth late on, before Spotland rose to acclaim the striker when he was substituted seconds later.
It was a small mercy for United, who have a week to lick their wounds before taking on another promotion rival, Burton Albion, at the Kassam Stadium.
Oxford Utd: Clarke, Hunt, Raynes, Mullins, Wright, Potter, Wroe, Rose, Williams (Kitson 58), Smalley, Connolly (Constable 67).
Unused subs: Henly, Long, Davies, Ruffels, Rigg.
Rochdale: Lillis, Rafferty, Lancashire, O'Connell, Done, Cavanagh, Vincenti, Allen, Rose, Henderson, Hogan (Donnelly 85).
Unused subs: Thomson, Eastham, Kennedy, Gray, Porter, Bunney.
Referee: Eddie Ilderton (Tyne & Wear).