Ethical Standards Command will investigate the death of a man while he was being transported to the Brisbane Watchhouse last night.

The 68-year-old man was arrested at a home in Fortitude Valley at around 7pm in relation to alleged drug offences and transported to the Brisbane City Watchhouse.

Upon arrival at around 7.17pm, the man was found unresponsive in the police vehicle.

Police rendered first aid and Queensland Ambulance Officers attended, but he was pronounced dead a short time later.

His cause of death is yet to be determined.