We examined the effect of constant-release melatonin capsules on the physiology and morphology of female collared lemmings exposed to either chronic long (22L:2D) or short (8L:16D) photoperiod, or to a change in photoperiod. When animals were maintained on unchanging long or short photoperiod, subcutaneous melatonin implants were without effect. However, when animals were reared on either 22L:2D or 8L:16D and transferred to the alternate photoperiod at weaning, melatonin (implanted at weaning) prevented most photoperiod-related responses. At sacrifice (after 8 weeks of treatment), melatonin-implanted animals exposed to a change in photoperiod did not differ from animals remaining in the original photoperiod with respect to pelage color, bifid claw size, uterine mass, or serum prolactin (PRL). In contrast, regardless of treatment, animals exposed to a photoperiod transfer developed a body mass that partially or fully reflected that characteristic of the secondary photoperiod; i.e., both control- and melatonin-implanted animals transferred from long to short photoperiod developed a large body mass. These results indicate that masking the endogenous melatonin rhythm via constant-release melatonin implants renders collared lemmings unable to respond to a change in photoperiod with respect to most physiological parameters. However, the striking seasonal change in body mass experienced by collared lemmings appears to be at least partially independent of a melatonin signal.