Previous evidence has demonstrated a relationship between the pineal organ and carbohydrate metabolism in goldfish Carassius auratus. Pinealectomy in this species causes a decrease in liver-glycogen stores, disappearance of the diurnal variation in liver-glycogen concentrations, and an increase in plasma glucose. These effects occur independently of photoperiod acclimation and are seasonal in nature. Results presented in this paper suggest that the pineal organ may regulate carbohydrate metabolism by altering insulin responsiveness in these animals. Goldfish that are pinealectomized during the spring resist insulin-induced hypoglycemia more than sham-operated animals. This insulin resistance is not induced by pineal-organ removal during late summer. Glucose-tolerance tests of pinealectomized animals during midwinter and late summer indicate that pineal-organ removal had little or no effect on insulin activity during these months. The seasonality of these responses coincides with the seasonality of the known relationship between pineal organ and carbohydrate metabolism. The insulin resistance induced by pinealectomy during the spring may account for the metabolic effects of pineal-organ removal.