Detectives are appealing for witnesses following an assault on Catherine Street in Limavady during the early hours of Sunday morning.
It was reported that at around 1.30am a 27-year-old man had been assaulted.
He is currently being treated in hospital for a serious head injury.
A 36-year-old man has been arrested and is in custody assisting police with their enquiries.
Detective Sergeant Brian Reid appealed for anyone who witnessed the incident or anyone who may be able to assist the investigation in any other way to contact detectives in Limavady on the non-emergency number 101.
If someone would rather provide information without giving their details they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555111.