Concentration of androgens, binding-sites of androgen-receptors as well as the activity of 5 alpha-reductase of the ventral prostates have been studied in the zinc-deficient rats. Eight-week-old Sprague-Dawley rats were housed individually in cages and divided into two groups, I. Experimental group, redivided into 2 sub-groups: (a) maintained on a zinc-deficient diet for 3 and 6 weeks, (b) zinc-deficient diet for 3 weeks then re-fed normal diet, II. Control group fed with normal diet. The ventral prostates were dissected free and homogenized after the rats sacrificed. Androgens and androgen-receptor were quantified by RIA and "exchange assay" respectively. The result obtained from the zinc-deficient rats were significantly different (P less than 0.05) from those of the control group 1. The zinc-deficient 3 and 6 weeks rats showed the plasma testosterone concentration decreased by 18% and 15%. 2. The cytosolic (fmo1/mg protein) dihydrotestosterone and its binding receptor were decreased by 10-15% and 28-30%, respectively. 3. The nuclear (fmo1/mg DNA) dihydrotestosterone and its receptor protein were ony 2-3% and 3-3.6% of the control group respectively. 4. Studies on the androgen metabolism showed that the metabolite, dihydrotestosterone (fmo1/mg protein) of rats with zinc-deficient 6 weeks was only 48% of the control group. 5. The prostate weight (mg) of zinc-deficient rats was 45-50% of the control group.(ABSTRACT TRUNCATED AT 250 WORDS)