The team initially said DeRosa was released Saturday evening, but said Sunday that he was hospitalized overnight at Banner Desert Medical Center in Mesa as a precaution.
DeRosa is expected to be examined by a cardiologist Monday and could be back on the field by the middle of the week, the Cubs said.
His irregular heartbeat was not life-threatening, according to the Cubs.
DeRosa, who turns 33 on Tuesday, began complaining of a rapid heartbeat during groundball drills Saturday morning. After being examined in the clubhouse by a team doctor, he was taken to the hospital in an ambulance.
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.
He began his major league career with Atlanta in 1998. DeRosa also played for the Texas Rangers in 2005 and 2006.
