Two series of experiments were performed in parallel on the isolated perfused rat pancreas. The experimental conditions differed only as pertaining to temperature. In one series the organ and the perfusion liquid were maintained at 37.5°C and in the other at 28°C. The pancreases were perfused from the start of the experiments with a perfusion medium containing 8.3 mmol/l glucose. The effects of various stimulatory agents were studied (glucose 16.6 mmol/l, tolbutamide 0.4 mmol/l, acetylcholine 0.5 μmol/l, glucagon, 2.8 nmol/l, and L-isoprenaline 0.05 μmol/l). At 37.5°C the insulin secretion induced by high glucose or tolbutamide, acetylcholine, and glucagon was biphasic and not statistically different. In all cases the hypothermia (28°C) decreased insulin secretion. However, glucoseinduced and tolbutamide-induced insulin secretion was more decreased than the secretion induced by acetylcholine and glucagon. The study of the secretion ratios obtained at 28°C relative to 37.5°C showed that the ratios for the glucose and tolbutamide groups were significantly lower than those obtained for acetylcholine and glucagon gsroups for both the first and the second phase. The ratios were not significantly different between glucose and tolbutamide on the one hand and acetylcholine and glucagon on the other hand. In all groups the ratios 28°/37.5° for the second phase were lower than those obtained during the first phase. L-isoprenaline induced only a weak increase in insulin secretion and this was not long lasting; this increase was not statistically different at both temperatures.