Melatonin affects food intake, body mass and adiposity of several mammals, but the effects of melatonin on energy metabolism remain largely unknown. This study investigated subacute effects of persistent melatonin treatment and continuous light on carbohydrate and fat metabolism of rat liver and kidney. The male and female rats (no.=40) were maintained either in 12L:12D photoperiod or in constant light. Half the rats in both lighting conditions were treated with continuous-release melatonin implants. Liver lipid concentrations, liver and kidney glucose-6-phosphatase, glycogen phosphorylase and lipase esterase activities, glycogen contents as well as plasma T4, T3, insulin, glucose and melatonin concentrations were determined. There was clear sexual dimorphism in the responses to exogenous melatonin and constant light. Continuous light stimulated carbohydrate metabolism of rat liver. Exogenous melatonin enhanced utilization of liver carbohydrates but suppressed hepatic lipolysis. Changes in normal circulating melatonin concentrations led to enhanced utilization of kidney carbohydrates supporting a role for melatonin in renal function. Both exogenous melatonin and constant light seem to have a strong regulatory effect on rat energy metabolism.