Male SPF bred Wistar rats were adapted to natural light (N) and to a 12:12 h (light-dark) artificial light (A) regimen in the course of the year. The rats were analyzed at 3 h intervals during 24 h approximately at the time of the vernal and autumnal equinox and at the winter and summer solistice. Serum insulin circadian oscillations depended on the season, being different in various light regimens. The mesors were the highest during summer, the lowest during winter in both regimens. The external acrophases of insulin in the N differed from those in the A group, contrary to the computative ones. The annual mean of serum insulin concentration was lower in the N than in the A group. The circadian oscillations of corticosterone were influenced primarily by the time of year. The mesors were the highest during summer, lower in winter and spring in N and A group. The computative acrophases were similar in both groups in all seasons except spring. The external acrophase was similar in both regimens during the year. The response of insulin, a major anabolic hormone, to various light regimens during the day and year was different from that of corticosterone, a major hormone of the stress reaction.