Abstract: A closure assembly (11) for a baby's bottle (10) comprising a regulator cap (16) which is adapted to be mounted to the bottle (10) and having one or more apertures (22), a teat collar (17) threaded to the bottle and moveable between an open and closed position and a teat (18) coupled to the teat collar (17) and having an outlet (45) for passage of fluid, the teat having (17) a base flange (38) adapted for cooperation with the regulator cap (16) such that in the open position of said teat collar (17) fluid can pass through the one or more apertures (22) and through the teat outlet (45) and air can pass between the base flange (38) and regulator cap (16) into the bottle (10) through one or more of the apertures (22). In the closed position, fluid is prevented from passage through the one or more apertures (22) into the teat (16) and air is prevented from passing between the base flange (38) and regulator cap (16) into the bottle (10).