Previous evidence has indicated that plasma concentrations of insulin and free fatty acid (FFA) play crucial roles in regulation of hepatic triglyceride (TG) secretion. To further define the relationship between these variables, we have assessed the effect of pre-existing ambient insulin levels on the ability of perfusate FFA concentrations to stimulate TG secretion by perfused livers from five groups of rats. The groups and their mean plasma in vivo insulin concentrations were as follows: sucrose-fed (49 microU/ml), control (35 microU/ml), moderate streptozotocin-induced insulin deficiency (24 microU/ml), semi-starved control (11 microU/ml), and severe streptozotocin-induced insulin deficiency (6 microU/ml). Hepatic TG secretion by perfused rat livers from all five groups was stimulated by increments in perfusate FFA concentration. However, the magnitude of the stimulation of hepatic TG secretion by any given perfusate FFA concentration varied considerably among the groups, and appeared to be directly related to the height of the in vivo insulin concentrations. These results indicate that TG secretion by perfused rat livers is related to perfusate FFA concentrations, but emphasize the importance that pre-existing ambient insulin levels have in modulating the quantitative nature of this relationship.