Because the pineal hormone melatonin has been implicated in affecting adiposity in rats and fatty acid transport in certain rat tumor models, we tested whether melatonin regulates lipolysis in a normal cell system in vitro. Adipocytes were isolated from the inguinal fat pads (i.e. sc fat) of Sprague Dawley male rats during mid-light phase. Lipolysis was stimulated with isoproterenol (3 microM), and cells were incubated for 4 h in the presence or absence of a physiological circulating concentration of melatonin (1 nM). Lipolysis was measured by determining the amount of glycerol present in the incubation buffer, expressed as nmol glycerol/mg cellular fatty acid. We observed a 20- to 30-fold stimulation of basal lipolysis by isoproterenol, and this stimulation was inhibited 50--70% by melatonin. Melatonin exhibited this effect over a wide range of concentrations tested (100 pM-1 microM) with an IC(50) of approximately 500 pM. The effect by melatonin (1 nM) was completely blocked by pertussis toxin (50 ng/ml), by 8-bromo-cAMP (10 nM), and by the melatonin receptor antagonist S-20928 (1 nM). These results suggest that the antilipolytic effect occurs through one of the G(i) protein-coupled melatonin receptors because we have shown that both the mt(1) (Mel 1a) and MT(2) (Mel 1b) melatonin receptors are expressed in inguinal adipocytes. Melatonin inhibition of lipolysis was not observed in adipocytes isolated from rat epididymal fat pads (i.e. visceral fat), even though these cells also express both the mt(1) and MT(2) receptors. The results indicate that physiological circulating concentrations of melatonin inhibit isoproterenol-induced lipolysis in rat adipocytes via a G protein-coupled melatonin receptor-mediated signal transduction pathway in a site-specific manner.