This study aimed to determine the influence of the pineal gland on plasma aldosterone and some electrolyte levels in the rat. The rats were exposed to constant light and given exogenous melatonin. Plasma aldosterone, Na, K and CI levels were compared between the light exposed group, light exposed plus exogenous melatonin given group and controls. Exposure to constant light caused a decrease in plasma aldosterone, Na, CI and increase plasma K levels, and exogenous melatonin administration prevented these effects in vitro. In conclusion, these changes in plasma aldosterone and electrolyte levels can be attributed to the adrenocortical stimulatory effect of melatonin.