A receptor protein that selectively binds oestrogens has been demonstrated in the cytosol of the canine prostate gland. The steroid–receptor complex was found to have a sedimentation coefficient of 4–5 S with respect to bovine serum albumin after sucrose density-gradient centrifugation. The high affinity and low capacity of the protein for oestrogens was indicated by displacement studies, which gave a value of 3·8 ± 1·53 (s.d.) × 10−10 mol/l for the dissociation constant. A metastasizing prostatic tumour was also shown to possess this receptor, with binding properties similar to those exhibited by the receptor in normal prostatic cytosol. The implications of these findings are discussed with regard to normal prostatic function in the dog and the virtually inevitable advent of prostatic hyperplasia with age in this species.