A hydraulic model was designed to study the effects of converging streams such as occur in patent ductus arteriosus. The results showed that flow across the fistula was dependent primarily on the resistance at the outlets of the “systemic” as against the “pulmonic” circulation. Even very slight differences in resistance could effect very large shunts. When the left ventricular output was great, the loss across the shunt was relatively unimportant. However, when the left ventricular output was low, most of its flow passed across the shunt and the systemic delivery virtually ceased. This situation became much more severe if the systemic resistance was raised even slightly above that of the pulmonic outflow path.