Quassia amara is a medicinal plant with several pharmacological properties [1-3]. There are preliminary reports implicating this plant in male reproductive toxicology. Its bioactive principle quassin has been reported as the antifertility agent. However, the mechanism of action remains to be elucidated, but the major site of action appears to be the epididymis. The action of Q. amara and quassin on epidydimal reproductive parameters was therefore investigated. Effect on sperm cells in vitro was also examined. Q. amara extract (100mg kg-1 p.o.), quassin (0.1 and 2mg kg-1 p.o.) and distilled water (0.5ml p.o.) were administered daily for 6 weeks to male rats (180-200g, n=5), and thereafter sacrificed [4]. Organ weights, sperm, hormonal and biochemical analysis were conducted. Sperm capacitation and acrosome reaction were determined by Coomassie brilliant blue staining technique. Values are Mean ± SEM, compared by ANOVA. In vitro extender studies were conducted on graded doses of the crude extract (50-500 µg/ml). The respective weight (g) of testes (0.78±0.14; 0.81±0.15; 0.99±0.14 vs 1.18±0.02), caudal epididymis (0.15±0.02; 0.12±0.02; 0.14±0.02 vs 0.30±0.01), seminal vesicle (0.9±0.18; 0.21±0.09; 0.28±0.09 vs 1.6±0.01) and prostate (0.15±0.02; 0.16±0.02; 0.16±0.01 vs 0.29±0.02); %sperm motility (42.0±10.2; 68.0±3.74; 68.0±3.74 vs 93.75±0.97), %viability (88.0±2.55; 88.6±3.39; 94.2±1.8 vs 97.25±0.75), count (58±4.24; 94±7.89; 88.8±5.61 vs 121.2±4.24) and volume (5.15±0.004; 5.14±0.02; 5.16±0.02 vs 5.20±0) were significantly reduced (p<0.05) in all treatment groups, except %viability for quassin high dose. No effect was observed on the weight, histology of visceral organs and liver function biomarkers. Lipid profile of the extract treated group were not affected, while quassin significantly reduced HDL (0.3±0.03; 0.2±0.02 vs 0.6±0.08, p<0.05) and increased LDL (0.9±0.04; 1±0.09 vs 0.7±0.07; p<0.05). Total epididymal protein were significantly reduced (0.67±0.11; 0.60±0.07; 0.70±0.17 vs 1.65±0.32, p<0.05). Q. amara extract significantly reduced (p<0.01) LH (0.6±0.14 vs 1.5±0.18) and testosterone level (0.03±0.01 vs 0.32±0.09), while quassin reduced FSH level (1.6±0.19; 2.2±0.2, p<0.05). In vitro extender study to verify these results showed that the action of the extract on sperm parameters is consistent, as progressive motility declined and abnormal morphology increased with dose. Also, after in vitro induction of sperm capacitation, the percentage of “acrosome reacted capacitated sperm cells” was significantly lower (27.2±1.98; 24.0±2.74; 37.8±10.72 vs 95.2±0.86, p<0.01) in all treated groups. Q. amara and quassin may have reproductive specific toxic effect and its deleterious effect on sperm cells affects capacitation and acrosome reaction.