To study the effect of long-term use of GnRH agonists on the number and size distribution of ovarian follicles, two experiments were performed on adult female rats. Treatment was continued for 52 days in Experiment 1 and for 229 days in Experiment 2. Every sixteenth section from one ovary of each rat was examined using a light microscope attached to a BioQuant image analysis computer system. In Experiment 3, control and previously treated rats were mated with known male breeders and the number and normalcy of the offspring evaluated. The results indicate that in rats treated with GnRH agonist: 1) the total number of follicles, the number and percentage of follicles less than 35 μm in diameter were significantly higher than in the control group; 2) the number and percentage of follicles greater than 50 μm in diameter were significantly lower than control; and 3) the number of rats that got pregnant and the litter size were not significantly different from control. We conclude that GnRH agonists inhibit the physiologic process of follicle recruitment and loss and that fertility is preserved after long-treatment administration of GnRH agonists.