OBJECTIVE To investigate effects of quercetin on weight and histology of testis and accessory sex organs and on sperm quality in adult male rats. METHODS Male Sprague-Dawley rats were injected s.c. with quercetin at the dose of 0, 30, 90, or 270 mg/kg body weight/day (hereafter abbreviated Q0, Q30, Q90 and Q270, respectively), and each dose was administered for treatment durations of 3, 7 and 14 days. RESULTS From our study, it was found that the effects of quercetin on reproductive organs and sperm quality depended on the dose and duration of treatment. After Q270 treatment for 14 days, the weights of testes, epididymis and vas deferens were significantly increased, whereas the weights of seminal vesicle and prostate gland were significantly decreased, compared with those of Q0. The histological alteration of those organs was observed after Q270 treatment for 7 days as well as 14 days. The sperm motility, viability and concentration were significantly increased after Q90 and Q270 injections after both of 7 and 14 days. Changes in sperm quality were earlier and greater than those in sex organ histology and weight, respectively. CONCLUSIONS Overall results indicate that quercetin might indirectly affect sperm quality through the stimulation of the sex organs, both at the cellular and organ levels, depending on the dose and the duration of treatment. Therefore, the use of quercetin as an alternative drug for treatment of male infertility should be considered.