This study examines the possibility that polymorphonuclear leukocyte activation, which can cause endothelial injury, may contribute to the capillary closure of diabetic retinopathy. To examine diabetes-related alterations in polymorphonuclear leukocyte activation, we compared the production of superoxide radical by these cells from normal and from diabetic cats that were maintained hyperglycemic. Polymorphonuclear leukocytes isolated from five diabetic and five normal cats were stimulated with 10 ng ml-1 phorbol myristate acetate, and the maximum rate of their superoxide radical production was measured spectrophotometrically. Stimulated polymorphonuclear leukocytes from diabetic cats generated more superoxide radical, at significantly higher rates, than did those from normals (3.32 +/- 0.33 and 2.50 +/- 0.41 nmol O2- min-1 10(-6) cells, respectively; P < 0.02). While addition of insulin or glucagon did not alter stimulated polymorphonuclear leukocyte radical production, glucose in high concentration did mildly impair its production in both groups. The exaggerated respiratory burst of polymorphonuclear leukocytes in diabetes could contribute to microvascular injury in the retina as well as in other tissues.