A consistent defect in follicle-stimulating hormone (FSH) secretion is seen in humans with Polycystic Ovarian Syndrome (PCOS); therefore, we evaluated whether Metrodin (a highly purified urinary FSH) administration concurrent with cyst induction or following cyst induction inhibits estrogen-induced cyst development and augments ovarian follicular growth in an established guinea pig model. All animals in these studies received subcutaneous implants containing oestradiol-17 beta (E2)-filled Silastic capsules for a 48-hour period. Guinea pigs in study #1 were administered four 0.25 mL injections of FSH or placebo at twelve-hour intervals simultaneously with the E2 treatment; guinea pigs assigned to study #2 were administered four 0.25 mL injections of FSH or placebo at twelve-hour intervals following the induction of the cystic condition by E2. Exogenous FSH appears to negate cyst formation when superimposed upon the cyst-inducing agent (E2). Further, treatment with FSH augmented the number of mid-sized follicles in both paradigms. This study is the first to establish evidence of an anti-cystic effect of FSH in an animal model.