The serum concentration of testosterone (T) 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-diol), 5 alpha-androstane-3 beta , 17 beta-diol (3 beta-diol), luteinizing hormone (LH), follicle stimulating hormone (FSH), and prolactin in young (less than 40 months) beagles with spontaneous benign prostatic hyperplasia (BPH) were not significantly (P greater than 0.25) different from serum hormone levels of age matched controls, or old (greater than 60 months) beagles with BPH. However, the serum hormone levels of age matched controls, or with BPH (2.46 +/- 0.51 pg per ml) was significantly (P less than 0.05) lower than that of age matched controls (3.93 +/- 0.35 pg per ml) and of old beagles with BPH is not associated with increased serum concentrations of androgens, or changes in serum levels of gonadotropins, but may be related to some change in estrogen biosynthesis or metabolism. In the second part of the experiment, castrate and intact beagles were treated for 4 months with three weekly injections of T, DHT, 3 alpha-diol, estradiol, T + estradiol, DHT + estradiol, or 3 alpha-diol + estradiol. As expected, the serum concentration of T, DHT, 3 alpha-diol, estradiol, LH, and FSH varied dramatically depending on the steroid regimen used. Analysis of these results showed that prostate weight was increased by steroid treatments which elevated the serum DHT concentration. However, glandular hyperplasia, indistinguishable from that seen in dogs with BPH, occurred only when estradiol was added to those steroid treatments which caused elevated serum DHT concentration.