Cubs spokesman Peter Chase announced DeRosa's release shortly after 5 p.m. Sunday. The team initially said DeRosa was released Saturday evening, but later said he was kept overnight at Banner Desert Medical Center in Mesa as a precaution.
DeRosa is expected to be examined by a cardiologist Tuesday to talk about treatment options -- including surgery, the Chicago Sun-Times reported.
His irregular heartbeat was not life-threatening, according to the team.
"He had an episode yesterday of an atrial dysrhythmia that he's had in the past that has lasted briefly," Cubs doctor Stephen Adams said, speaking with reporters Sunday afternoon at DeRosa's request. "This one had a bit of an extended period of time for which he contacted our head athletic trainer, Mark O'Neal, who appropriately saw him and was with him."
DeRosa began complaining of a rapid heartbeat Saturday morning during ground ball drills. After an exam by a team doctor, the Cubs called 911.
Some teammates stopped by Saturday to check up on DeRosa.
"Jason [Marquis] and I went right after practice, and hung out for a couple hours," Cubs shortstop Ryan Theriot told MLB.com on Sunday. "Everything was back to normal and he seemed fine. He was cracking jokes in there. Obviously, it's a scary thing but he seemed fine. I offered for him to come and hang out at the house and play guitar with the kids."
The incident did scare Theriot, though.
"He seemed more shaken up and a little scared about it," Theriot told the Web site. "When I read the reports [streaming] on ESPN, it scared me and I was there the whole time."
DeRosa is projected to be the Cubs' starting second baseman.
DeRosa batted .293 with 10 home runs and 72 RBIs in 149 games for the Cubs last season. They signed him to a $13 million, three-year contract in November 2006.
Information from The Associated Press contributed to this report.