ABSTRACT
Rats maintained on either constant illumination [LL] or constant dark [DD] were compared to those exposed to 14 hr dark Schedule [LD]. At the estrous phase, LL caused a reduction in LH level throughout the twelve weeks observation period. FSH levels were increased to a peak at the 8th week then decreased. Pituitary gland weight was significantly increased from the 8th week observation period onward with a histochemical presence of more FSH and LH producing cells. Ovarian and uterine weights were also increased with no evidence of ovulation histologically in the ovaries. Constant dark caused prolongation of the estrous cycle to 6-7 days in 96% of animals. At the estrous phase, LH level insignificantly increased throughout the observation period whereas FSH reached a peak maximum at the 8th week then decreased. No significant changes in pituitary, ovarian and uterine weight or cytology occured. This study demonstrates that the light-darw cycle as an environmental factor is profoundly affecting the reproductive cycle in female rats