A drain assembly for a urostomy pouch or other similar collection device for permitting continuous overnight draining of the pouch into a suitable receptacle, thereby allowing the user to wear the pouch in bed without the risk of fluid leakage or the inconvenience of interrupting sleep for periodic emptying of the pouch. The assembly includes a body member adapted to be secured to a pouch at the lower end thereof and defining a path for the outflow of the fluid contents of the pouch, a rotary valve member for selectively opening and closing the flow passage, and a drainage tube equipped with a coupler for releasably engaging a stem portion of the valve member.