The rat prostate carcinoma (Dunning R-3327) contains a relatively high concentration of androgen receptor (80-150 fmol/mg cytosol protein). This receptor was compared with androgen receptors in normal organs of the rat. Binding of testosterone and dihydrotestosterone was of high affinity (Ka = 2 .times. 109 M). Rates of dissociation were slow (t1/2 [half-time] testosterone = 60 h; t1/2 dihydrotestosterone = .apprx.160 h). At low ionic strength, the receptor was in an 8S form which dissociated to 4.5-5.0S in the presence of 0.4 M KCl. Fractionation of [3H]dihydrotestosterone cytosol by chromatography on phosphocellulose yielded a single peak of radioactivity eluting at 0.2 M Cl-. Determination of size at high ionic strength by gel filtration chromatography and sucrose gradient centrifugation indicated a Stokes radius of 53 .ANG. and sedimentation coefficient of 5S (MW 115,00). Cytosols occasionally yielded a 2nd peak of radioactivity eluting from phosphocellulose at 0.29 M Cl-. This fraction contained a smaller receptor [36 .ANG., 3.6S (MW 55,000)]. Both receptor forms were observed in cytosols from the normal dorsal prostate. The larger high-salt form of the receptor is identical to native androgen receptor in normal tissues. Smaller receptor forms in the tumor and in normal androgen-responsive tissues result from proteolytic cleavage of the native receptor during extraction.