We examined the differential change in body composition in response to a gradual reduction in both environmental temperature and photoperiod to mimic seasonal fluctuations in the wild (summer-winter transition), from ambient to 5degreesC and 1:23 light:dark for 8 weeks. In contrast to acute cold exposure used in previous studies, cold-acclimated rats showed an initial increase in growth rate relative to normothermic controls, possibly due to cold-stimulated hyperphagia. In hamsters, maintenance of growth rate during initial cold exposure reflects the intrinsic high oxidative capacity, while subsequent cessation of growth is consistent with the preparation for hibernation. Cold-induced atrophy of skeletal muscles coincided with increased capacity for non-shivering thermogenesis (NST) associated with a greater mass of brown adipose tissue (BAT). Cardiac hypertrophy may compensate for an increase in total peripheral resistance and/or work load of heart in both species (40% and 20%, respectively), while hypertrophy of lung (20% and 40%) and diaphragm muscle (7% and 40%) was consistent with increased ventilation associated with a cold-induced increase in basal metabolic rate. Gonadal atrophy in hamsters (160%) may be an energy saving strategy during the nonreproductive season, while maintenance of other endocrine (thyroid, adrenal, pineal) gland masses reflects the continued importance of hormonal regulation of homeostasis. The interspecific differences appear to accommodate the increased demands of shivering thermogenesis (skeletal muscle hypertrophy) or NST (BAT, diaphragm) in rats and hamsters, respectively. Those systems representing cardiovascular and metabolic control completed their adaptation quickly (within 4-week cold acclimation), while the respiratory and reproductive systems continued to respond to a further 4-week exposure. This differential time course may reflect the relative strength of selection pressure on these systems for the process of cold acclimation. (C) 2002 Elsevier Science Ltd. All rights reserved.