Canine prostatic DNA synthesis was evaluated by measuring [3H]thymidine incorporation into DNA of tissue slices in vitro. Among untreated beagles, prostatic DNA synthesis rates in young dogs with normal prostates, young dogs with spontaneous benign prostatic hyperplasia (BPH), and old dogs with BPH were 676 +/- 186, 1,220 +/- 156, and 641 +/- 88 cpm/100 micrograms DNA/hr, respectively. Among 81 young beagles (intact or castrated) that had been treated for 4 months with various steroids, rates of DNA synthesis varied according to the type of hormonal treatment. Prostatic DNA synthesis (cpm/100 micrograms DNA/hr) was significantly different (P less than 0.001) for dogs treated with estradiol alone (1,658 +/- 221 cpm/100 micrograms DNA/hr; n = 10 dogs), androgen alone (testosterone, 5 alpha-dihydrotestosterone, or 5 alpha-androstane-3 alpha,17 beta-diol; 1,000 +/- 61 cpm/100 micrograms DNA/hr; n = 31 dogs). However, there was no correlation between prostate size and rate of DNA synthesis (cpm/100 micrograms DNA/hr). Although treatment with estrogen alone resulted in the highest rate of DNA synthesis, it produced squamous metaplasia and the smallest prostates; these results are indicative of a high rate of cell turnover. Comparing prostates that reached the same size following 4 months of treatment with androgen alone or androgen plus estrogen, the rate of prostatic cell turnover was lower in the androgen plus estrogen group. These results are interpreted to indicate an inhibitory effect of estradiol on the rate of cell death in the presence of androgens.