Riordan, 82, who suffered a mild heart attack last December, said he became lightheaded and was in pain when he called the paramedics to his Brentwood home last Tuesday.
"They did an EKG and could find nothing wrong, but they insisted I be hospitalized," Riordan said. "They took me to St. John's, where my doctors came to see me and insisted I stay overnight."
After more tests, he said he was released on Wednesday and even was able to exercise that afternoon.
"It was all precautionary and I'm feeling fine now," Riordan said.
Opponents of his City Charter amendment, he said, are using the incident as an example of city services that could be impacted if Riordan's measure is approved by voters.
He said his proposal is designed to protect services to keep all the city money from going to pension costs.
Riordan is in the process of collecting the more than 245,000 signatures needed to place his measure on the March 5 ballot.