Pineal glands of male Syrian hamsters stimulated in vivo with isoproterenol (ISO) for 4 h before the onset of darkness showed a 4-h advance in the timing of the nighttime increases in both N-acetyltransferase activity and melatonin levels. When ISO (1 mg/kg) was administered every 2 h to animals kept in light during the night, a significant increase in melatonin synthesis was observed after 4-6 h. The results suggest that the Syrian hamster pineal gland can respond in vivo to continuous beta-adrenergic stimulation, but a lag period of 4-6 h is required before there is an increase in melatonin synthesis.