PURPOSE: Adverse sexual experiences such as erectile dysfunction (ED), loss of libido, and ejaculation disorders have been consistent side effects of 5-alpha reductase inhibitors (5ARI). The aim of this study was to evaluate the effects of 5ARI (finasteride) treatment on semen parameters and contraction of the corpus cavernosum and seminal vesicles in male rabbits. MATERIALS AND METHODS: Adult male New Zealand White rabbits (n=10) were randomized into 2 groups: finasteride-treatment (5ARI) group and vehicle-treatment (control) group. The 5ARI group received daily oral finasteride (10 mg/day) by gavage for 4~6 weeks, and the control group received the same concentration of the vehicle. The semen volume and semen parameters between the 2 groups were compared; thereafter, contraction or relaxation responses of smooth muscle strips of the corpus cavernosum and seminal vesicles were observed in an organ bath. RESULTS: Semen magnesium (14.2 vs 5.1 mg/dl) and protein (2.2 vs 1.6 g/dl) concentrations were significantly lower in the 5ARI group than in the control group. The concentrations of other parameters such as electrolytes (Na/K/Cl), fructose, and citrate did not differ between the 2 groups. The contractile responses to norepinephrine (NE) significantly increased in the 5ARI group compared to the control group and the relaxation responses to acetylcholine (ACh) or sodium nitroprusside (SNP) also increased in the 5ARI group. The contractile responses of the seminal vesicular strips to NE significantly decreased in the 5ARI group compared with the control group. CONCLUSIONS: The results suggest that finasteride may decrease contraction of seminal vesicle tissue and alter semen parameters. The effect of finasteride on erectile tissue was double-faced; enhancing both contraction and relaxation. Further study is needed in this respect.