Zambrano got the final out in the seventh before allowing one run on two hits with one strikeout and one walk in the eighth inning of the Cubs' 5-1 win over the Milwaukee Brewers.
"I felt good, that's it," was Zambrano's only comment after the game.
Zambrano had started 241 games since his last relief appearance on June 28, 2002.
He also added a sacrifice fly Saturday.
"I was glad to see Big Z come in, and as the inning progressed he threw the ball better and better," Cubs manager Lou Piniella said.
Piniella said before the game that he wouldn't hesitate using Zambrano in consecutive games, so it's possible Zambrano will see some action Sunday as the Cubs try to sweep the series.
Piniella, who wanted to bolster his struggling bullpen, converted Zambrano into a reliever last week after a loss dropped his record to 1-2 with a 7.45 ERA. There seemed to be conflicting reports as to whether Piniella and Zambrano are on the same page regarding whether the move is temporary.
ESPNChicago.com's Bruce Levine contributed to this report.