The purpose of this study was to test the effects of recombinant rat interferon-gamma (IFN-gamma) on rat pineal function in organ culture. The glands were pre-incubated with IFN-gamma (50, 100, and 1,000 antiviral units/ml) for two hours, followed by a stimulation with isoproterenol (ISO, 10(-6) M) (in the presence of IFN-gamma) for an additional three hours. The effect of IFN-gamma alone on the pineal function was also tested by adding another group incubated with IFN-gamma (1,000 antiviral units/ml) without ISO stimulation. The following parameters were estimated at the end of the incubation: the activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), the levels of melatonin, 5-hydroxytryptophan (5HTP), 5-hydroxytryptamine (5HT), and 5-hydroxyindole acetic acid (5HIAA), and the 5HT/5HIAA ratio. IFN-gamma had no effect on either NAT or HIOMT activities in the pineal glands. At 50 units/ml, IFN-gamma increased the melatonin content over the ISO-stimulated level in the pineal but not in the media. IFN-gamma alone, at 1,000 units/ml, also increased the melatonin content significantly over the control level. The levels of 5HTP and 5HIAA were significantly suppressed by IFN-gamma, either alone or in combination with ISO. The level of 5HT was not altered by any of the treatments. The 5HT/5HIAA ratio was significantly increased by IFN-gamma, either when given alone or when given in combination with ISO. The results suggest that IFN-gamma enhances melatonin production in the pineal gland by suppressing the oxidative deamination of 5HT to 5HIAA and shunting the biosynthetic pathway toward melatonin production.