The effect of insulin was studied on blood glucose, and on the glycogen level of liver, muscles and heart in fed and in starved catfish (Ictalurus melas). Fish received intraperitoneally 60 iu/kg body weight of bovine insulin, or physiological saline and were sacrificed after 2, 4, 8, 24, 72 hr from injection. Insulin caused a decrease of blood glucose level, both in fed and in fasted animals, and the effect is more evident in fed animals. After insulin treatment, liver glycogen shows a decrease which is significant at the 8th and 24th hr in fasted and in fed animals respectively; after 72 hr the glycogen level in livers of fed and fasted animals is still very low. Insulin increases the glycogen level both in white and in dark muscle, both in fed and in fasted fish, although with different characteristics, but at the 72nd hr in all animals, the increases are significant. Hormone treatment does not change heart glycogen levels in fed catfish till the 24th hr, then there is a net decrease; in starved animals the decrease begins at the 2nd hr, but only at the 48th hr is it significant. The role of insulin was discussed in relation to the lowering of glycogen concentration in liver, in connection with the fact that many authors found different and even opposite effects of this hormone in various fish. It is possible that the glycogen depletion observed in liver after insulin injection is not due to a direct action of this hormone, but depends on the stimulated production of other specific glycogenolytic hormones, such as epinephrine and/or glucagon.