Back ground: Many animals use the length of the day (photoperiod) to predict and adjust to seasonal changes in the environment through predictive changes in physiology and behaviour. The present work is designed to study the effect of different photoperiod regimes, i.e., short photoperiod 10L:14D, long photoperiod 14L:10D and 12L:12D light-dark cycle (LD) on the structure and hormonal secretion of the thyroid gland and ovaries in adult and aged female rats.   
Material and methods: The rats were divided into 6 groups, each group contained 6 adults (6 months old) or aged rats (18 months old) and maintained for one month under different photoperiod regimes.
 Results: Short and long photoperiods increased the activity of the thyroid gland of the adult rats. This gland was full of small follicles lined with simple columnar epithelium. Serum thyroxin levels were extremely higher in long photoperiod group than that observed in 12:12LD and short photoperiod groups. In contrast short photoperiod induced an increase in serum thyroxin level in the aged rats. Short photoperiod and 12:12 LD cycle enhanced gonadal activity of the adult rats. The ovary contained clusters of primordial follicles, different stages of ovarian follicles, and, few numbers of atretic follicles. In the groups of the aged rats all the stages of the ovarian follicles exhibited signs of atresia in such groups. LH, FSH, progesterone and estrogen were increased mainly in the control group in adult and aged rats compared to the photoperiod groups. From the present result we could conclude that:  12:12 light dark cycle enhanced the structural and functional activities of ovaries in adult rats, while aging decreased their activities. In contrast, long photoperiod induced thyroid gland activities   in adult rats while short photoperiods activated the gland of aged rats.