SUMMARY: An isolated rat liver perfusion system was used to study the effects of insulin and glucagon on renin substrate production. Normal livers synthesized renin substrate at a rate of 28·3 ± 3·8 (s.e.m.) ng angiotensin I equiv./g liver each h (n = 8). The addition of insulin (more than 0·1 i.u.) to the perfusion significantly enhanced the production of renin substrate which was about twofold higher than normal control values (P< 0·001). However, glucagon (20 μg) did not affect the synthesis of renin substrate. These results indicated that insulin promoted the synthesis of renin substrate by the isolated rat liver.