The purpose of this study was to compare the response of Syrian hamster pineal glands in organ culture either to isoproterenol, a β-adrenergic agonist, or to dibutyryl cyclic AMP. When pineal glands were collected at night, hamsters were exposed to light for 30 min to depress pineal N-acetyltransferase (NAT) activity and melatonin values to low levels. Pineal glands were removed and placed in organ culture containing either isoproterenol or dibutyryl cyclic AMP and subsequent changes in NAT activity and melatonin levels were measured. At night, isoproterenol (10–7 or 10–6M) induced an increase in the NAT activity and melatonin levels in both pineals and culture media. However, dibutyryl cyclic AMP was either ineffective or minimally effective in stimulating these parameters at either different incubation times (2,4, and 6 h) or drug concentrations (0.1, 0.5, and 1.0 mM). Conversely, when rat pineal glands were incubated with either isoproterenol (10–7) or dibutyryl cyclic AMP (0.5 mM) dramatic rises in NAT activity and melatonin levels were observed. In another experiment, hamster pineal glands were collected from animals killed either late in the light period (19.00 h) or in the latter half of the dark period. Isoproterenol promoted NAT activity and melatonin production only in glands collected in the latter half of the dark