Insulin-like effects of N-succinyl-L-trialanine p-nitroanilide-hydrolyzing protease (STA-protease) purified from Pronase were investigated in rat epididymal adipose tissue. The enzyme stimulated the conversion of [3-³H] glucose into lipid and suppressed the epinephrine-stimulated lipolysis in the fat cells, like trypsin and insulin. When the fat pads were incubated with STA-protease in the presence of glucose, pyruvate dehydrogenase activity in the homogenate of the incubated fat pads was stimulated markedly. In the absence of glucose, STA-protease did not stimulate pyruvate dehydrogenase activity though trypsin and insulin showed a slight but significant stimulation. Further, the stimulatory effect of STA-protease in the presence of glucose was inhibited by the addition of 3-O-methylglucose or phlorizin to the incubation medium of the fat pads. Trypsin and insulin still showed a significant stimulation under similar conditions. When the homogenate of intact fat pads was incubated with STA-protease, no stimulation was observed throughout the range of effective concentrations. These results suggest that STA-protease also reacts with the cell surface and consequently mimics the actions of insulin, though a slight difference between STA-protease and trypsin may exist as regards the mechanism of stimulation of pyruvate dehydrogenase.