• It has been suggested that the pupillary response to hydroxyamphetamine hydrobromide eyedrops will separate distal from proximal lesions of the sympathetic pathway from the brain to the eye (the pupil fails to dilate in postganglionic lesions). We studied the pupillary response to hydroxyamphetamine in two groups of rabbits with surgically produced Horner's syndrome. Rabbits in one group had postganglionic lesions. In all animals, a miotic pupil developed ipsilateral to the surgical lesion, and oculosympathetic paresis was confirmed in each by testing pupillary response to topically applied cocaine. In rabbits with postganglionic lesions, hydroxyamphetamine failed to dilate the miotic (Horner's) pupils as well as it dilated the normal pupils, while in rabbits with preganglionic lesions both miotic and normal pupils dilated equally. Hydroxyamphetamine appears to be a diagnostically useful drug in Horner's syndrome.