In 10 healthy volunteers we investigated the effects of intraduodenal oleic acid at various concentrations (0-40 mM) and at various degrees of emulsification on pancreaticobiliary secretion and the release of secretin and cholecystokinin (CCK) into plasma. We found that the release of the two hormones was directly related to the dose of fat and to the degree of emulsification. The threshold of CCK release (and amylase output) was low in comparison with the threshold for secretin release (and bicarbonate or volume output). When the degree of emulsification of the fat was increased, no simple relation was observed between hormone levels and pancreatic exocrine secretion. The output of bile salts was identical at various fat concentrations. We conclude that both secretin and CCK are dose-dependently released by emulsions of oleic acid in normal man and that the thresholds for release are probably different.