The present paper describes some physicochemical properties of the soluble 3α‐oxidoreductases in the rat testis and prostate, and comparison with rat epididymal 3α‐oxidoreductase, published previously (Hastings&Hansson 1979). The testicular enzyme shows properties very similar to that in the epididymis (size, stability, pH optium) except for minor differences in charge (iso‐electric point). The prostatic enzyme revealed a slightly higher molecular weight, and was more sensitive to heating than those in the testis and epididymis, whereas the iso‐electric point was the same as that in the testis (pI‐5.25). The enzymes in all tissues exhibit very similar shapes (f/fo 1.14‐1.17).The similar properties of the testicular and prostate 3α‐oxidoreductases to those previously reported for that in the epididymis may indicate that these enzymes represent identical peptide chains. The small differences observed in size, temperature stability and change may be due to their presence in differ