It follows an alleged incident that took place in Cardwell Road, Gourock, at 12.30am yesterday.
The alleged incident left a Gourock man with serious head injuries and he was rushed to Glasgow's Southern General Hospital where he underwent surgery yesterday.
He is now said to be in a serious but stable condition.
Detective Sergeant Gerard McWatt, of Greenock CID, issued an appeal for information yesterday and asked witnesses to come forward.
Cardwell Road was cordoned off with police tape for much of yesterday and uniformed officers remained on scene hours after the attack.
Police were also seen speaking to residents for information as they worked to piece together what happened during the incident.