Drugs acting on androgen receptors modify opioid transmission in the central nervous system. To investigate a direct interaction, we studied whether the binding of [ 3 H]diprenorphine to mouse brain membranes was modified by cyproterone acetate (progesterone derivative with antiandrogen activity), flutamide (non-steroidal antiandrogen), 5α-dihydrotestosterone and progesterone. Only cyproterone acetate inhibited [ 3 H]diprenorphine binding (IC 5 0 = (1.62 ± 0.33) 10 - 6 M) without modifying its association rate. These results suggest that cyproterone acetate binds to opiate receptors independently of its classical androgenic intracellular receptor effect.