Ronaldo was undergoing medical tests for AC Milan on Friday following the Italian club's offer to Real Madrid for the Brazilian star.
Ronaldo spent the night in a Milan hotel and was undergoing tests at a private clinic near Como, news agency ANSA reported. Further exams will be done later at the club's training center in Milanello, the report said.
Milan confirmed tests were under way but did not give details.
Milan has made an undisclosed offer for the 30-year-old striker and club vice president Adriano Galliani has said he will travel to Madrid on Monday in the hope of completing the signing.
Ronaldo left the Spanish capital by private jet on Thursday to attend Milan's 2-2 draw wth AS Roma in the first leg of an Italian Cup semifinal at San Siro stadium.
Italian sports daily Gazzetta dello Sport said the deal had not been sealed yet, with Milan offering euro7 million (US$9 million) for the player and Madrid asking euro7.5 (US$9.7 million).
Much will depend on the results of the medical tests, the paper said.
Ronaldo has six days remaining in the January transfer window to switch clubs. Otherwise, he will have to stay at Madrid at least until the end of the season.
The forward, who hasn't been picked for Madrid since the January 7 defeat at Deportivo La Coruna, said on Monday his difficulties with coach Fabio Capello seem set to force him out of the club.
A move to Milan would take Ronaldo back to the city where he spent five years with rival Inter Milan. He joined Madrid in 2002, winning the Spanish league at the end of his first season - his last major title, the AP reports.
The player had earlier attended training at Real Madrid's Valdebebas complex as usual.
He is no stranger to the Italian city having played for AC Milan's city neighbours Internazionale before his move to Madrid.
Ronaldo, 30, left Inter in 2002 shortly after inspiring Brazil to World Cup glory.
Much of his time with the Nerazzurri had been blighted with injury but he blamed poor relations with then Inter coach Hector Cuper for his decision to move to Madrid.
"Everything, then, will depend on the results of the medical. There are still a lot of things to do." said Milan vice-president Adriano Galliani as he confirmed that a formal bid has been put forward for the World Cup winning striker, itv-football.co.uk says.
Prepared by Alexander Timoshik