The purpose of the present study was to examine the effects of administration of sublethal doses of carbaryl on nighttime rat pineal melatonin synthesis in the presence and absence of propranolol, a β-adrenergic receptor antagonist. Two groups of adult male albino rats were administered orally N-methyl-l-naphthylcarbamate (carbaryl) (8. 33mg/kg BW daily in corn oil) for six successive days; another two groups received corn oil only.On the last day of carbaryl treatment, half of the animals received an intraperitoneal injection of propranolol (20 mg/kg body weight, one hour before lights off). The other two groups were given a saline injection. Four hours after darkness onset, pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) activities as well as pineal concentrations of 5-hydroxytryptophan (5HTP), serotonin (5HT),5-hydroxyindole acetic acid (5HIAA) and pineal and serum melatonin levels were measured. Nocturnal NAT activity was increased due to carbaryl administration but the pesticide was ineffective in stimulating NAT activity in rats treated with propranolol.Pineal 5HT was decreased due to carbaryl administration but 5HTP and 5HIAA levels were unaffected. Pineal and serum melatonin levels were decreased due to propranolol treatment. The results indicate that carbaryl may influence pineal NAT activity by a mechanism that involves β-adrenergic neural transmission. The purpose of the present study was to examine the effects of administration of sublethal doses of carbaryl on nighttime rat pineal melatonin synthesis in the presence and absence of propranolol, a β-adrenergic receptor antagonist. Two groups of adult male albino rats were administered orally N -methyl-l-naphthylcarbamate (carbaryl) (8. 33 mg / kg BW daily in corn oil) for six successive days; another two groups received corn oil. Only the last day of carbaryl treatment, half of the animals received an intraperitoneal injection of propranolol (20 mg / kg body weight, one hour before lights off). The other two groups were given a saline injection. Four hours after darkness onset, pineal N-acetyltransferase (NAT) and hydroxyindole- O-methyltransferase As well as pineal concentrations of 5-hydroxytryptophan (5HTP), serotonin (5HT), 5-hydroxyindole acetic acid (5HIAA) and pineal and serum melatonin levels were measured. Nocturnal NAT activity was increased due to carbaryl ad ministration but the pesticide was ineffective in stimulating NAT activity in rats treated with propranolol.Pineal 5HT was decreased due to carbaryl administration but 5HTP and 5HIAA levels were unaffected. Pineal and serum melatonin levels were decreased due to propranolol treatment. influence pineal NAT activity by a mechanism that involves β-adrenergic neural transmission.