Ronaldo was taken to a clinic in Frankfurt late Wednesday night after complaining of dizziness.
Ronaldo went through a series of medical tests and doctors found no abnormalities with the striker, who returned to the team hotel and was cleared to practice on Thursday, the Brazilian Soccer Confederation said.
"Ronaldo is fine. He just fell a bit dizzy, nothing serious," CBF spokesman Rodrigo Paiva told The Associated Press. "He was taken to the clinic for tests and everything was OK."
Ronaldo underwent a muscle training session on Thursday morning and was feeling fine, Paiva said.
"He was taken to the clinic just as a precaution," he added. "The results of the tests showed right away that there was nothing wrong with him. Doctors were not worried."
Brazilian doctors were expected to talk to the media about Ronaldo's condition later Thursday. It wasn't immediately clear whether Ronaldo would speak.
The 29-year-old Real Madrid forward fell ill only a few hours after returning to the team's training camp in Koenigstein, near Frankfurt, after his dismal performance in Brazil's opening 1-0 win over Croatia on Tuesday.
Ronaldo was substituted in the 69th minute against Croatia, having had one shot on goal in the match.
The Brazilian squad had been given the day off from training on Wednesday and was not due to play again until the Group F match against Australia in Munich on Sunday.
The incident was just the latest setback for Ronaldo, who is now in danger of losing his position as an indisputable starter for Brazil.
Ronaldo won finals in 1994 and 2002 -- when he was the leading scorer at the World Cup co-hosted by South Korea and Japan -- and he could join Pele as the only player on three World Cup-winning teams. Pele played on Brazil's teams in 1958, 1962 and 1970.
Ronaldo came close to a third title in 1998, but he had unexplained convulsions on the day of the final. He played poorly, and Brazil lost 3-0 to host France.
Ronaldo has twice had serious knee injuries that have sidelined him for several months at a time.
After a series of injuries with Real Madrid earlier in the year, Ronaldo arrived for Brazil's training camp admittedly overweight and not in form. Even the Brazilian president, Luiz Inacio Lula da Silva, got involved in the weight controversy.
In Brazil's last warmup match before its World Cup debut, against New Zealand in Switzerland, Ronaldo was forced to leave the field because of blisters caused by defective shoes, and a couple of days later he missed practice because of a mild respiratory infection.