We examined whether the repeated oral administration with a high dose of L-3-(3,4-dihydroxyphenyl)-alanine (L-DOPA) in 0.5% carboxymethyl cellulose increases serum luteinizing hormone (LH) levels in male rats. Serum LH levels were increased 4 h after a single administration of 1000 mg/kg L-DOPA to male rats, and returned to control levels within 8 h after administration. Four hours after a single administration, serum LH levels were significantly increased by L-DOPA at 1000 mg/kg, but not at 20, 100 or 200 mg/kg. Decreases in body weight and relative weight of the prostate were observed after 7 and 14 days of administration of 1000 mg/kg per day L-DOPA, but no changes were observed in weight of the testis, epididymis or seminal vesicle. The administration of L-DOPA at 500 or 1000 mg/kg per day for 7 or 14 days resulted in increased basal serum LH levels and decreased basal serum prolactin levels 24 h after the last administration. Serum testosterone levels tended to be higher in treated than in control rats. The levels of two metabolites of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in rats treated with 500 mg/kg per day tended to be slightly higher than those in control rats after 7 days of administration. Levels of DA, DOPAC and HVA were significantly increased after 7 and 14 days of administration of 1000 mg/kg per day and after 14 days of administration of 5000 mg/kg per day. The level of norepinephrine, but not its metabolite 3-methoxy-4-hydroxyphenylglycol, was significantly increased after only 7 days of administration of 1000 mg/kg per day. No significant changes were observed in levels of 5-hydroxytryptamine or its metabolite 5-hydroxyindole-3-acetic acid with administration of 500 or 1000 mg/kg per day. These findings suggest that a prolonged treatment with a high dose of L-DOPA in male rats induces release of LH from the pituitary, resulting in sustained elevation of LH levels in peripheral circulation.