Temporal profiles of the amount of vasoactive intestinal polypeptide (VIP) were examined in the medium of organotypic suprachiasmatic nucleus (SCN) slice cultures over a 2-day period. Arginine-vasopressin (AVP) level was also measured in the same medium. The slices of the SCN were obtained from 7-8-day-old rats and cultured individually in tubes on a roller drum for 14 days. The VIP amount in the medium of SCN culture showed a circadian rhythm with a approximately 22-h period. Circadian rhythms with identical periods were also observed in AVP amount of the same culture. However, the peak time of the VIP rhythm was slightly ahead of that of the AVP rhythm. Furthermore, the total VIP amount in the medium over a 24-h period was six times as large as that of AVP. These results suggest that there is a circadian rhythm of VIP which is released from the ventrolateral SCN.