Real Madrid started talks with representatives from AC Milan on Wednesday to discuss a possible deal that would see the three-times World Player of the Year Ronaldo return to Italy this season.
Real agreed to meet Milan vice-president Adriano Galliani and sporting director Ariedo Braida before their pre-season match against Anderlecht at the Bernabeu and talks are expected to continue late into the night.
Real Madrid president Ramon Calderon said on Tuesday he would meet representatives from the Italian club.
The nine-times European champions are reported to want around 25 million euros for the Brazil striker, who joined them for 45 million ($57.70 million) from Milan's city rivals Inter in August 2002.
Real are aiming to get a preferential option to buy Milan's Brazilian midfielder Kaka included in the deal.
Ronaldo, who will turn 30 next month, has notched up 100 goals since joining Real and has been the top scorer in each of his four seasons at the club.
Real's decision to sign Dutch striker Ruud van Nistelrooy from Manchester United and the arrival of Italian coach Fabio Capello are believed to have played an important part in the willingness of both the player and the club to agree to a move.
Milan received an fresh injection of cash following the sale of Andriy Shevchenko to Chelsea for an estimated 45 million euros and qualified for the group phase of the Champions League on Tuesday.