Collared lemmings undergo several photoperiod-mediated seasonal physiological changes. When exposed to short photoperiod, lemmings increase in size, develop a bifid claw, and molt to a white pelage. Previous data indicate that body mass, claw size, and pelage color are influenced by hormones of testicular origin, suggesting that, on a seasonal basis, changes in production of, or sensitivity to, testicular hormones may play a role in the development of the phenotype characteristic of the ambient photoperiod. The present study was designed to determine if the active testicular hormone(s) is testosterone (T) and/or estradiol (E2) and if seasonally changing physiological traits in female lemmings are also influenced by gonadal status. Fifty-day-old lemmings, reared in 22L:2D (long day), 16L:8D (intermediate day), or 8L:16D (short day), were either gonadectomized or sham operated and given either empty Silastic implants or implants containing T (4 or 10 mm; castrated males). E2 (4 mm undiluted or diluted 1:4 with cholesterol; ovariectomized females), the aromatase inhibitor ATD (androsta- 1,4,6-triene-3,17-dione; 2 x 20 mm; intact animals of both sexes), or ATD plus a 10-mm T implant (castrated males). After a 6-week treatment period, changes in body mass, bifid claw width, pelage color stage, and serum prolactin (PRL) were assessed. The effects of gonadectomy and steroid treatment depended upon photoperiod. Whereas gonadectomy increased mass gained by both sexes under intermediate and short day, under long day only females showed the positive mass response to gonadectomy. Treatment with T and E2 reversed the effect of gonadectomy on body mass under intermediate day and decreased the amount of mass gained under short day. Treatment with ATD (males) and E2 (females) indicated that E2 was the hormone responsible for the tonic, inhibitory effect of the gonads on body mass in both sexes. Claw size was most sensitive to steroid manipulation in animals housed in long day, in which all treatments had a negative influence. Gonadectomy under short day resulted in the development of a whiter pelage in both sexes. The effect of gonadectomy on pelage in female lemmings, and its reversal by E2 treatment, may have been partially due to alteration of serum PRL. © 1994 by Academic Press, Inc.