The effect of somatostatin, an inhibitor of release of a number of gastrointestinal and other hormones, on choleresis was investigated in chronic, bile fistula dogs with taurocholate-stabilized bile flow. Somatostatin inhibited both fasting and meal-stimulated choleresis, and bile flows during somatostatin inhibition of both fasting and fed dogs were similar, suggesting a complete suppression of factors causing feeding choleresis. Although a transient decrease in bile salt output was observed, bile salt output was unaffected during most of the period of bile flow inhibition. Hormone suppression by somatostatin, indicated by measurement of serum insulin, occurred over a similar time course as inhibition of choleresis. These observations provide further evidence for physiological humoral regulation of choleresis.