Authorities say crews were called in just before 2 a.m. That's when smoke was spotted coming from the stadium.
The fire apparently started near a fryer at Friday's restaurant, and then moved into the ventilation system.
It took about an hour to put the fire out.
Fire officials say there is some smoke and fire damage in the restaurant area. Some meeting rooms, elevators and escalators were also flooded.
No one was hurt.
Brewers officials say Monday night's game between the Brewers and the Cincinnati Reds will go on as scheduled.