A detailed description of the water flow through the ventilation system of the bullfrog tadpole has been facilitated by recording the time dependence of normal hydrostatic pressures in the system. The mouth and nasal intakes are discontinuous but a pharyngeal pressure pump to support the buccal pressure pump has been found to assist in maintaining a flow through the gill cavity. The alternate action of these pumps drives water into the gill cavity during almost the entire ventilation cycle and owes its efficiency to the occlusion between the buccal cavity and pharynx during the compression stroke of the pharyngeal pump. The need for a valvular function of the ventral velum has thus been demonstrated as it is the only structure between the buccal cavity and pharynx which can effect such cyclic occlusion.