Castrated sexually inactive rats were treated with the synthetic androgen 17β-hydroxy-17α-methyl-estra-4,9,11-triene-3-one (methyltrienolone, R 1881), testosterone benzoate (TB), 5α-dihydrotestosterone benzoate (DHTB) or oestradiol benzoate (OB) and tested for sexual behaviour. The synthetic androgen, R 1881, was as potent as TB in stimulating sexual behaviour and OB stimulated all aspects of copulatory behaviour but DHTB had no significant effect. The synthetic androgen, R 1881, is not metabolized by target cells and rapid metabolism could explain the failure of DHTB to stimulate sexual behaviour.