Abstract 1. 1. Nalorphine and nalorphine-7,8-oxide (nalorphine epoxide) behaved as partial agonists on the kappa-receptor in the electrically stimulated mouse vas deferens. 2. 2. The effects of a GTP-analogue, GppNHp and Na + on the inhibition of [ 3H]ethylketocyclazocine binding by dynorphin 1–13, nalorphine, its epoxide and naloxone (antagonist) were studied with a synaptosomal fraction of guinea pig brain (except a cerebellum) and compared with the intrinsic activity of the test drugs, which was estimated in the electrically stimulated mouse vas deferens. 3. 3. The effects of GppNHp and Na + on the affinity of the drugs to the kappa-receptor correlated with their intrinsic activities.