The Walmart in Glen Carbon was evacuated about 4:30 p.m. Saturday after reports of an odor and reopened at 6 p.m.
The store, which is currently undergoing reconstruction to convert it to a Super Walmart, is at 400 Junction Drive.
The Glen Carbon Fire Protection District responded to the scene, as well as police and ambulances.
"We got a report of a strange odor and several people were complaining of feeling ill," said Glen Carbon Fire Protection District Chief Ralph Well. "We were unable to find or identify anything hazardous."
The store was cleared and reopened by 6 p.m. No injuries were reported.