Seasonal changes in the testicular activity were studied in the male Greater Yellow bat, Scotophilus heathi, in Varanasi, India (25"N, 83"E). The seasonal pattern of body weight of male bats was also recorded and compared with circulating androstenedione (A) and testosterone (T) concentration. There was a gradual increase in the body weight of the bat from September onward mainly due to accumulation of adipose tissue. Finally, body weight reached a peak level in November. Two periods of peak testicular weight and serum T concentration but only one peak of serum A concentration were observed. The first peak of T coincided closely with the peak A concentration and increased body weight. The first peak of T occurs several months before the time of mating and ovulation. The second peak of T correlated closely with the period of mating. In vitro study showed seasonal variation of the testicular responsiveness to LH stimulation. In vitro study also showed two peaks in the T but only one peak in the A production by testes in response to LH during different phases. Preliminary study showed that IGF-I but not insulin augmented hCG induced T production by testes in vitro. Our observation of an increase in serum A concentration during the period of overfeeding and weight gain and decline in A during the period of weight loss in S. heathi adds strength to the concept that nutritional status can influence hormonal function. This may be the reason for asynchrony between male and female gametic cycle found among vespertilionid bats.