The aim of the present study was to determine the serum levels of insulin and corticosterone during different reproductive stages in a natural population of the male bat, Scotophilus heathi and their relationship to body weight and androstenedione level. Changes in body weight were marked by weight gain before winter dormancy and weight loss during winter dormancy. Circulating insulin level varied significantly over the season and correlated positively with changes in body weight and androstenedione level. Circulating corticosterone level also varied significantly over the season but correlated negatively with changes in body weight and androstenedione level. High corticosterone level during August coincided with a period of increased feeding activity in this species. The results suggest that insulin may act as an anabolic agent to promote fat deposition and corticosterone exerts a predominantly fat-mobilizing influence. A high insulin level prior to winter dormancy may be an important factor responsible for inducing high androstenedione concentration shown in Scotophilus heathi.