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Specific binding of [³H]19-nortestosterone in the 100000 g cytosol of the rat bulbocavernosus/levator ani muscle (BCLA) and prostate was demonstrated by agargel electrophoresis at low temperature and compared qualitatively and quantitatively with the binding of tritiated testosterone and 5-dihydrotestosterone (5-DHT). Both tissues showed a greater binding affinity for 5-DHT than for 19-nortestosterone, with testosterone binding the least well of the three. The relative binding affinities in the BCLA and prostate were: 19-nortestosterone: testosterone = 1·4, 19-nortestosterone: 5-DHT = 0·7. The differences were statistically significant (P < 0·02). The concentrations of receptor sites for 5-DHT were 171 ± 20 (S.D.) fmol/mg prostatic cytosol protein and 24 ± 4 (S.D.) fmol/mg BCLA cytosol protein. The in-vitro metabolism of the three steroids in both tissues was also investigated by thin-layer chromatography. After incubating for 2 h at 0 °C the prostate was shown to reduce 26% of the 5-DHT to androstanediols whilst the BCLA showed a 5% conversion. Testosterone was converted by the prostate to 5-DHT (10%) and the androstanediols (6%) whilst the BCLA showed little activity in this respect. Comparing these in-vitro data with in-vivo findings from the literature, in both organs there is a positive correlation of the extent of binding in vitro to the stimulation of growth in vivo, bearing in mind that testosterone is metabolized to 5-DHT in the prostate whilst in the BCLA, 5-reductase is essentially absent.