Anti-androgenergic agents are usually used for patients with benign prostatic hypertrophy (BPH). However steroidal anti-androgenergic agents tend to suppress the sexual function. This side effect is very significant in middle-aged men. Therefore we studied the preventive effect of indeloxazine hydrochloride (INDX), which induces an increase of the dopamine level in the brain, on the sexual dysfunction induced by an anti-androgenergic agent (allylestrenol: ALE). Thirty-six patients with BPH were classified into two groups, one used ALE only, and the other ALE with INDX. For the subjective evaluation of the sexual function, a self assessment questionnaire method was employed before and after administration. We especially studied 3 questions, "morning erection", "erectile capacity" and "frequency of sex". For the objective evaluation of the sexual function, nocturnal penile tumescence (NPT) was measured using an erectometer. NPT occurs in healthy males as a physiological phenomenon and it shows the erectile capacity objectively. The levels of LH, total testosterone and free testosterone were also determined. In the ALE only group, sexual dysfunction was found subjectively and objectively, but in the ALE with INDX group, it was not found. Levels of LH, total testosterone and free testosterone were decreased in the both groups. There was no significant difference between the two groups. We hypothesized that the sexual dysfunction due to ALE is related with not only to the decrease of androgen, but also to suppression of the central nervous system; for example, the suppression of the area of the brain mediating sexual behavior.(ABSTRACT TRUNCATED AT 250 WORDS)