The response of conjunctival vascular dilatations to topical epinephrine was tested in 25 subjects. Irregular venular dilatations disappeared, venular microaneurysms underwent partial contraction, while capillary microaneurysms did not contract following application of epinephrine. Biopsy specimens of the conjunctiva were examined by light and electron microscopy in four diabetic patients in whom microaneurysms had been demonstrated by biomicroscopy. Marked thickening of the basement membrane and enlargement of the endothelial cytoplasm were noted in the dilated capillaries. Pericytes were normal in number and appearance. Conjunctival capillary dilatations in three nondiabetics failed to show these changes. A milder degree of basement membrane thickening was observed in a younger diabetic. The finding of conjunctival capillary microaneurysms in a diabetic would thus appear to indicate the presence of microangiopathy.