The ocular hypotensive effects of medetomidine, a relatively selective alpha 2-agonist, and its analogs were tested in rabbits and cats and their inhibition of adenylate cyclase in the isolated bovine ciliary process was also studied. It was found that topical unilateral administration of medetomidine (0.5-2.0%) to the normotensive rabbits produced a dose-dependent bilateral decrease in IOP with peak reduction in IOP at 2 hr in the treated eye and 1 hr in the untreated eye. A dose-dependent mydriasis was also observed in the treated eye. The dose-response curves of medetomidine and its analogs showed that the ranked order of intrinsic activity at lowering IOP was medetomidine greater than or equal to MPV-1440 greater than detomidine greater than MPV-1441 and MPV-305 BII. At concentrations lower than those used in rabbits, topical application of medetomidine to the normotensive cats lowered IOP in both treated and untreated eyes. Medetomidine and detomidine caused a dose-dependent inhibition of isoproterenol-stimulated adenylate cyclase activity. Detomidine was found to be a partial agonist producing about 43% of maximum inhibition obtained by medetomidine. The IOP efficacy of these alpha 2-agonists paralleled their effects on adenylate cyclase activity. The results demonstrated that these imidazoline derivatives are effective ocular hypotensive agents which may be useful in understanding the contribution of alpha 2-receptors to the regulation of IOP.