The regulation of glycogenolysis and alanine and lactate gluconeogenesis, glycogenesis, and oxidation by porcine insulin was studied in isolated American eel hepatocytes. Experiments were performed in the summer, winter, and spring using naturally fluctuating water temperatures to establish the seasonal dependence of these processes and their hormone sensitivities. Porcine insulin (10(-8) M) maintained glycogen content, decreased total glucose production, increased lactate and alanine flux to glycogen in hepatocytes from summer and winter eels, and had a small stimulatory effect on alanine gluconeogenesis in the spring. The hormone counteracted bovine glucagon-stimulated glycogen depletion and glucose production, but only offset the glucagon effect on gluconeogenesis when glycogen content was below summer values. Effects of the two hormones on oxidation were additive in the summer, but were equivocal at other seasons. The magnitudes of the hormone effects on metabolism were generally smaller in the winter than in the other seasons. Anglerfish glucagon (10(-8) M) effects, studied in the spring, mimicked those of bovine glucagon. Porcine insulin effects in the presence of anglerfish glucagon were the same as in the presence of bovine glucagon. These studies generally support the antagonistic role between insulin and glucagon and the insulin-stimulated C3 precursor flux to glycogen reported in mammalian hepatocytes. Although these metabolic processes are seasonally adjusted, the precise mechanism involved is not understood.