Insulin and glucagon choleresis was studied in an in situ, isolated perfused guinea pig liver system. Glucagon caused a small, significant increase in bile salt independent flow (1.83 +/- 0.19 to 2.02 +/- 0.23 microliter g-1 min-1), and dose-related increments over 2-16 micrograms were observed. Insulin alone had no choleretic effect. However, the combination of insulin and glucagon caused a response (1.89 +/- 0.15 to 2.42 +/- 0.19) greater than glucagon alone, and insulin stimulated choleresis when glucagon was present in substimulatory amounts. These observations demonstrate direct effects of glucagon and insulin upon the bile secretory apparatus. Glucagon directly stimulates choleresis, while insulin acts more subtly by potentiation with glucagon.