In order to determine whether circulating melatonin (MEL) is necessary for the gonadal regression seen in hamsters exposed to a short photoperiod, animals were immunized with N-acetylserotonin (NAS) conjugated to protein. Antibodies formed against this conjugate bind MEL and NAS equally. When exposed to a short photoperiod (L:D, 1:23), a dramatic regression was seen in gonadal weight with no effect on body, Harderian gland or adrenal weight. Plasma testosterone levels also dropped. Immunization against NAS did not modify these effects, suggesting that circulating MEL does not mediate the antigonadal effect of the short photoperiod.