Effects of photoperiod, temperature and food and water availability on body and testicular mass of golden hamsters were investigated. Lack of water or food inhibited body growth. Short photoperiod (LD 8:16) enhanced body growth in males fed 'poor diet' but it had no effect in animals on 'high quality diet'. Cold exposure diminished body growth in animals with access to water but not in those deprived of water. It is suggested that lowered motor activity is mainly responsible for body mass gain of animals kept under the short photoperiod. All stressful factors (water deprivation, 'poor diet' or cold) induced involution of testes. The time course of testicular involution was similar to that induced by short photoperiod. Testicular involution was, after several weeks, followed by spontaneous recrudescence. Combination of stressful external cues speeded up testicular involution. Since the testicular involution induced by water deprivation, "poor diet" and cold also occurred in animals kept in constant light or under long photoperiod (which are known to produce functional pinealectomy) it is concluded that the inhibitory effects of stressful external factors on testes are not mediated by the pineal. The decrease in testosterone production induced by external cues precedes changes in testicular mass. In individuals kept in the cold, testosterone production remains low, in spite of spontaneous recrudescence of the testes. No simple relationship between testosterone concentration in the blood and testicular mass was observed. The mass of the interscapular brown adipose tissue (ISBAT) changes strikingly during the hibernation cycle. Immediately after exposure to cold it decreases and stays at the low level for 40 days.(ABSTRACT TRUNCATED AT 250 WORDS)