Female sheep were implanted with the anabolic agent trenbolone acetate and the effects of the treatment on the skeletal muscle androgen receptor studied. In two separate trials the binding capacity/mg skeletal muscle protein was reduced. In trial 1 the animals were implanted (80 mg trenbolone acetate) at the start and after 5 weeks of the trial and samples taken one week later while in the second trial samples were taken 15 days after a single implant (80 mg trenbolone acetate). The ability of a variety of steroids and steroid-like compounds to displace [3H] testosterone and [3H] methyltrienolone was also studied. The androgen receptor was also partially purified using heparin-sepharose chromatography The involvement of the anddrogen receptor in the mode of action of trenbolone is discussed.