Medetomidine (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole) was tested for alpha 2-adrenoceptor agonist activity and compared to several reference agents. In binding studies carried out with rat brain membrane preparations, medetomidine showed high affinity for alpha 2-adrenoceptors, as measured by the displacement of [3H]clonidine (Ki 1.08 nM compared to 1.62, 3.20, 6.22 and 194 nM for detomidine, clonidine, UK 14,304 and xylazine, respectively). The affinity of medetomidine for alpha 1-adrenoceptors, as measured by [3H]prazosin displacement, was much weaker, yielding a relative alpha 2/alpha 1 selectivity ratio of 1620 which is 5-10 times higher than that of the reference compounds. Medetomidine caused a concentration-dependent inhibition of the twitch response in electrically stimulated mouse vas deferens with a pD2 value of 9.0 compared to that of 8.6, 8.5, 8.2 and 7.1 for detomidine, clonidine, UK 14,304 and xylazine, respectively. The effect of medetomidine was antagonized by idazoxan. In anaesthetized rats, medetomidine caused a dose-dependent mydriasis which could be reversed by alpha 2-adrenoceptor blockade. In receptor binding experiments and isolated organs medetomidine had no affinity or effects on beta 1-, beta 2-, H1, H2, 5-HT1, 5-HT2, muscarine, dopamine, tryptamine, GABA, opiate and benzodiazepine receptors. Based on these results, medetomidine can be classified as a potent, selective and specific alpha 2-adrenoceptor agonist.