Naloxone hydrochloride eyedrops 1 mg/ml dilated the pupils of 36 out of 47 opiate addicts on methadone maintenance treatment, without precipitation of acute withdrawal effects, but not those of healthy unmedicated subjects. The response in addicts was attenuated by certain ancillary treatments and by withdrawal of methadone treatment. The size of the response suggests some potential clinical use for topical naloxone as a diagnostic test of current opioid influence and possibility of physical dependence. The local mydriatic response, which was restricted to the treated eye, indicates that the effect of opiates on the pupil in man is determined, at least in part, by a peripheral action.