SummaryCardiac output of adult White Pekin ducks restrained on their backs was determined by the dye dilution technique. Mean flows in ml/mm/kg were 286.8 and 253.4 for males and females respectively, and the difference was not statistically significant. Total peripheral resistance in units per kg averaged 0.56 for males and 0.62 for females and the difference was not significant. It is pointed out that on a body weight basis cardiac output in ducks is higher than in chickens and of about the same magnitude as for turkeys. TPR tends to be lower in ducks than chickens.