The Reds fight back against Pompey to claim a point, but it could probably have been more in this Sky Bet League One clash.
Yet another Cauley Woodrow strike an hour into this match got the Reds a point in a game they dominated at Oakwell.
That goal cancelled out Gareth Evans' first half opener, which was gift wrapped by Barnsley, two minutes before the break.
Reds Head Coach, Daniel Stendel had made four changes from the defeat at Wycombe, with Brad Potts, Dimitri Cavaré, Adam Jackson and Dani Pinillos back in the starting XI.
Barnsley were dominant for much of the first half, Woodrow in particular looked destined to add to his increasing goals tally, but found visiting 'keeper MacGillivray in inspired form.
The Pompey stopper thwarted Woodrow twice in the first 20 minutes of the game. The first effort a half-volley that was parried above him, the ball dropped a few yards from goal for Potts who slammed the effort over when it looked easier to hit the target.
Woodrow's second effort was hit with venom, following a superb Kieffer Moore cut-back, and again the 'keeper parried, but got enough on the save this time to see the ball behind for a corner.
On the half hour mark, Cameron McGeehan - who was impressive for the Reds - took aim from the edge of the box and saw the ball superbly tipped over by the Pompey goalkeeper with one hand.
Potts got to the byline with 38 minutes on the clock, his cross was punched by MacGillivray but only to Moore who instead of squaring for a teammate, tried the shot but it screwed well wide.
Against the run of play, the league leaders took the lead. Alex Mowatt tried to clear from inside his own box, but the clearance didn't get far, with Gareth Evans picking the ball up on the edge of the area and picking his spot, firing his shot in hard and low into the bottom right-hand corner of Adam Davies' goal.
HALF TIME: Barnsley 0-1 Pompey
Stendel replaced Cavaré at the break, with Jacob Brown on in his place at right back as the Reds looked to get back into the contest.
The second period was a lot less eventful, indeed it took 10 minutes to see an effort from either side, in what were atrocious conditions.
Potts it was who tried a long range strike, but the ball whistled a yard over the Portsmouth crossbar.
However, just five minutes later, Potts was involved in the Reds' equaliser, as his right-wing cross towards the near post was spotted perfectly by Woodrow who'd peeled off his marker, and the Fulham loanee stabbed the ball home beyond the onrushing Pompey 'keeper.
George Moncur made way for Lloyd Isgrove with 63 minutes on the clock.
Pompey had few chances, but on 69 minutes Ronan Curtis was sent through on goal, but his shot was superbly repelled by Davies with his legs.
Mamadou Thiam replaced Potts with 10 minutes remaining.
But whilst the Reds remained on top and looked most likely to grab a winner, clear cut opportunities weren't coming and despite Moore shooting wide when a brilliant Isgrove run had freed him inside the box, Barnsley had to make do with a point on the day.
FULL TIME: Barnsley 1-1 Pompey
Davies, Cavaré (Brown 45), Jackson, Pinnock, Pinillos, Potts (Thiam 80), Mowatt, McGeehan, Moncur (Isgrove 63), Moore, Woodrow.
Unused: Greatorex, Adeboyejo, Lindsay, Bähre.
Att: 12, 441 (1,674 away)