Baxter arrived near the end of practice Thursday in sweats and appeared groggy. He did not participate in any drills and appears questionable for Saturday's game at Oregon State.
"He came in very ill today and we had him checked out and he wasn't able to practice," Kiffin said.
Baxter, a top recruit out of Mission Bay High in San Diego, is USC's third-leading rusher this season, with 248 yards on 57 carries.
He missed the season opener against Hawaii due to suspension and was out with turf toe Oct. 16 against Cal, but had been participating fully in practice as late as Wednesday.
Kiffin said he remained hopeful Baxter would be able to make the team's 2 p.m. PT flight to Oregon on Friday.
"We'll keep working on him through tonight and tomorrow morning," he said.
Baxter was not made available for comment by school officials.
Pedro Moura is co-author of the USC blog on ESPNLosAngeles.com. Follow him on Twitter.