Collared lemmings (Dicrostonyx groenlandicus) of both sexes show seasonal changes in body mass and body composition. Previous studies using single- point sampling indicated that, in young males, these photoperiod-mediated changes are associated with changes in circulating growth hormone (GH), corticosterone (B), and thyroid hormones. The present study was conducted to (1) examine daily fluctuations in serum levels of GH, B, and thyroxine (T4) in animals exposed to long (22L:2D, 'LD'), intermediate (16L:8D, 'ID'), and short (8L:16D, 'SD') photoperiods, (2) confirm that conclusions based on single-point sampling are valid when photoperiod-related differences in hormone concentration are examined over 24 hr, (3) examine the effect of photoperiod on hormone concentrations in adults of both sexes, and (4) characterize the daily pineal melatonin rhythm in this species. Adult male and female collared lemmings housed in SD had higher levels of GH, and lower levels of B and T4, even when the diurnal variations in serum concentrations of these hormones were taken into account. A significant effect of time was observed on serum B (ID animals only) and serum T4. ID lemmings had B levels that were similar to those of SD animals, but concentrations of GH that more closely resembled those of LD animals. Females had lower GH and T4 than males. Pineal melatonin concentration closely tracked the dark phase of the day in each of the three photoperiods. Photoperiod-mediated changes in melatonin synthesis may mediate observed day length-related differences in serum concentrations of metabolic hormones, which in turn may contribute to the seasonal changes in body composition observed in collared lemmings.