Abstract:The purpose of this study was to compare the responses of the Syrian hamster and rat pineal glands in organ culture to vasoactive intestinal peptide (VIP). The endpoints in these studies were the activities of pineal N‐acetyltransferase (NAT) and hydroxyindole‐O‐methyltransferase (HIOMT), as well as pineal and medium melatonin levels. When rat pineal glands were incubated with either VIP (1 μM) or isoproterenol (1 μM), a β‐adrenergic agonist, a significant increase in NAT and HIOMT activities and melatonin levels were observed within 3 hr. Conversely, during the day, VIP (1 μM) was ineffective in stimulating these parameters in hamster pineal gland after incubation times of either 2, 4, 6, or 8 hr. In another experiment, hamster pineal glands were collected from animals killed in the late dark period (after 30 min light exposure). In these glands, isoproterenol promoted NAT activity and melatonin production; however, VIP was ineffective in stimulating either NAT or HIOMT activities; likewise, VIP had no stimulatory effect on pineal melatonin levels at night. Finally, when hamster pineal glands at night were incubated with either 0, 10 nM, 100 nM, 10 μM, or 100 μM VIP, no changes in any parameter of melatonin synthesis were measured. The results indicate that the hamster pineal gland, unlike that of the rat, may not respond to VIP with an increased melaton