The 21-year-old prospect surrendered five runs and retired just one of the eight batters he faced Friday as the Brewers dropped a 9-7 Cactus League decision to the Rockies in Scottsdale, Ariz.
With Zack Greinke sidelined with a broken rib, Peralta was 1-0 with a 4.66 ERA in his first four spring appearances. His first Cactus League start began innocently enough when he gave up a leadoff single to Seth Smith and retired Jose Lopez on a fly ball to center field.
But Carlos Gonzalez delivered an RBI double and scored on a base hit by Troy Tulowitzki. After Todd Helton walked, Ty Wigginton and Ryan Spilborghs hit back-to-back doubles to make it 5-0, and Peralta's day was over when he issued another free pass to Chris Iannetta.
"I don't know if he's amped up and he wants to show everybody," Brewers manager Ron Roenicke told MLB.com on Thursday. "I don't know. It's hard to tell. If he was, I can't blame him. He wants to show us something and he's just not locating his pitches."
Peralta pitched at two Minor League levels last season and was 8-6 with a 3.79 ERA in 27 games, including 25 starts.
Former first-round pick Greg Reynolds blanked the Brewers for three innings but ultimately was charged with six runs on nine hits over six innings.
Ben Paulsen, a 2009 third-round selection, gave the Rockies a 7-6 lead with a two-run single in the seventh.