Objective: Quassin, the bioactive constituent of Quassia amara plant, has been reported to decrease sperm count, sperm motility, normal sperm morphology and male fertility index. These sperm characteristics are however acquired in the epididyimis. Information on the effects of quassin on epididymal functions is scanty. The effects of quassin on epididymal functions were therefore investigated in Wistar albino rats.Methods: Quassin (0.1 and 2 mg/kg) and distilled water (0.5 ml) were administered by gavage (p. o) daily for 6 weeks to male rats (180-200 g, n=5), and thereafter sacrificed. Sperm motility, viability and count were examined microscopically. Serum from each rat was analyzed for Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH) and Testosterone by enzyme immunoassay technique. Alanine Aminotransferase (ALT), Aspartate Aminotransferase (AST) and Alkaline Phosphatase (ALP) were measured spectrophotometrically. Epididymal HongrES1 protein expression was assessed using immune histochemical technique. Data were analyzed using Student's t-test at p=0.05. Results:Quassin significantly decreased sperm motility, % viability and count. Serum FSH, LH and testosterone were also decreased. The epididymal tubules were coiled and hyperplastic, while no visible lesion was observed on the testes. Serum ALT, AST, LDL, HDL, cholesterol, triglyceride, bilirubin and total protein levels were not affected by quassin. Total epididymal protein was however significantly reduced while total testicular protein increased. Epididymal HongrES1 protein expression was suppressed by treatment with quassin. Conclusion:Quassin decreased epididymal tissue total protein and HongrES1 protein expression in rats. These effects indicate that antifertility action of quassin is pronounced in the epididymis.