Britain and Ireland's elite will be coming up against some of the world's biggest names, in challenging conditions and without any previous game time together, all of which makes this particular encounter far more dangerous than previous opening fixtures.
"It will be a different challenge, the humidity…it's the first hit out for these guys and they have some class players," said Rowntree, who believes The Barbarians will offer far more than a free-flowing attacking threat.
"We play them every year at Twickenham with England and we think it's going to be this razzmatazz team where they throw the ball around. But remember four years ago we were beaten?
"There's good enough players there to cause potential upset and they keep adding to it. It will be a good work-out for us and we are taking it very seriously."
Saturday's fixture will be the first chance for many of Gatland's pride to show their class in a Lions shirt and Rowntree knows that a great deal of responsibility comes with that honour.
The former Leicester prop toured twice as a player in 1997 and 2005 and was part of Sir Ian McGeechan's coaching staff in South Africa last time out.
He knows better than most that the current crop of Lions will have to concentrate for each and every one of the 'warm-up' fixtures, even though the primary objective is a first series victory since his debut Tour 16 years ago.
"It's our first game and the first opportunity for a lot of these guys in a Lions shirt. There are a lot of things that go with a Lions shirt that players have to get right. There is a lot of responsibility," added Rowntree.
"Regardless of whether it's a Barbarians team or not we will be taking it very, very seriously.
"There are potential banana skins in the schedule. We have to be switched on.
"It's the wrong end of the season for many of these guys so we've been very mindful of that and it means we have got to be very accurate in what we are doing."