The effects of hepatic vagotomy upon the circadian rhythm of lipogenesis and body fat store levels were tested in seasonally obese Syrian hamsters. Lipogenesis was studied 8 weeks after surgery by measuring the incorporation of label into epididymal and retroperitoneal fat pad lipid in animals killed 30 min after intraperitoneal [3H]-glucose injection and 2 h after bovine insulin injections. A marked circadian variation in insulin-stimulated lipogenesis was present in sham-operated controls. The peak of lipogenic activity occurred at light onset and was 3-fold greater than during the middle of the photophase. The peak in the circadian variation of plasma insulin coincided with the peak of lipogenic responsiveness to insulin in these animals. Hepatic vagotomy completely abolished the circadian variation in insulin-stimulated lipogenesis and reduced the magnitude of the daily lipogenic peak at light onset by 65%, but only slightly altered the phase of the circadian variation in plasma insulin by 4 h. Hepatic vagotomy did not influence plasma glucose levels, which did not vary as a function of time of day in either group. Body fat stores were severely reduced (40%) by hepatic vagotomy when examined 8 weeks after surgery. The present study demonstrates important roles for interactions of the circadian rhythms of insulin and tissue lipogenic responses to insulin in the regulation of body fat stores. Furthermore, this study demonstrates that hepatic vagal activity regulates the daily interval of lipogenic responsiveness to insulin and thereby directly influences body fat stores in the Syrian hamster.