A FRANTIC eight minutes at Oakwell in which the Reds conceded three goals ensured Portsmouth picked up all three points.
Pompey goals from Colby Bishop, Paddy Lane and Connor Ogilvie saw the away side 3-0 up in 17 minutes.
Goals from Barry Cotter and then Callum Styles weren’t enough to pull off a fight back.
Neill Collins named an unchanged side from the victory over Burton Albion at the weekend, though Callum Styles returned to the bench to replace Josh Benson.
It was the Hungarian international’s first feature in a match day squad since the transfer window ended.
He had missed the game against Cheltenham Town and then Burton Albion either side of playing in both of Hungary’s European Championship qualifiers.
Collins had said he had returned with an illness, but he recovered in time to feature on the bench.
Pompey almost went ahead with just 35 seconds on the clock after Liam Roberts’ pass was intercepted by the away side.
But smart defending from Jordan Williams and Kacper Lopata ensured the scores stayed level.
Pompey took the lead in the seventh minute after Colby Bishop was judged to have been brought down by Roberts in the box - he looked a yard offside.
It took just a minute for Pompey to double their lead.
This time Paddy Lane fired home after a great through ball from midfield.
There were questions of offside, with Barnsley fans in the East Stand unhappy with the linesman.
Fortunately for them, the sprinklers went off and he was sprayed with water.
But they weren’t laughing for long, as Connor Ogilvie made it 3-0 on 17 minutes.
He was found unmarked in the box and it was easy to find the back of the net.
Pompey fans began to ‘olay’ in the 23rd minute as they passed it round the Barnsley team like a training ground exercise.
The Reds went into the break 3-0 down, after having their worst performance in recent memory as the fans booed the players off.
Collins made three changes at the break, with Styles, Dodgson and Cosgrove replacing Russell, Cadden and McAtee.
Those subs paid off just three minutes after the restart as Sam Cosgrove did brilliantly to play through Barry Cotter, who found the back of the net.
Barnsley then came into the game, showing signs of confidence, but still needed Roberts to deny Bishop on a number of occasions to keep the score at 1-3.
Cole almost halved the deficit on 59 minutes, as he slid in on Will Norris - but his effort trickled just wide.
Styles pegged one bak in the 77th minute with a fantastic diving header from Owen Dodgson’s cross.
It sparked life into both the players and the fans as Oakwell erupted.
Cosgrove had a great chance in the 89th minute to level the game, but he leans back and his effort is well over the bar.
Oakwell erupted when the fourth official held up the board indicating there were to be seven minutes of added time.
But the score stayed 3-2 and the Reds lost their first game in four.