It has been reported that, in the rat, different types of stressful procedures increase daytime melatonin production. Electroconvulsive shock (ECS) has been shown not to affect daytime pineal melatonin in rats killed 9 hr after its administration. It is possible that the long-lasting interval between the ECS administration and the sacrifice of the animals obscured an effect of the stressful procedure. In the present study, the effects of acute ECS on daytime melatonin production were evaluated in both ECS- and sham-treated rats killed 30, 60, 120, and 240 min after treatment. As compared with the sham-treated animals, rats receiving acute ECS (80 mA, 0.5 sec) did not show any significant difference in either pineal or serum melatonin levels at any time point after treatment. These data indicate that single ECS does not affect daytime melatonin production in the rat and suggest that the pineal gland may respond differently to the various stressful procedures.