1. Defective leukocyte-endothelial interactions are observed in experimental diabetes mellitus. The present study investigated the effect of aminoguanidine, an inhibitor of advanced glycation end products formation, on leukocyte-endothelial interactions in alloxan-induced diabetic rats. 2. In rats anaesthetized with sodium pentobarbitone, the internal spermatic fascia was exteriorized and the microcirculation was observed by a closed-circuit TV coupled to a microscope. The number of leukocytes rolling along the venular endothelium and sticking to the vascular wall was determined after topical application of zymosan-activated plasma (1 mg ml(-1)), as well as the number of adherent and migrated cells after an irritative stimulus (carrageenan 100 microg). 3. The diabetic state decreased the number of rolling, sticking and migrated leukocytes. Pretreatment of diabetic animals with aminoguanidine (250 mg kg(-1) day(-1), for 18 days) normalized these values. To be effective, aminoguanidine had to be administered chronically, starting treatment before induction of the diabetic state. 4. The preventive effect was unrelated to the number of circulating leukocytes, or to the hyperglycaemia or to the hyperosmolality secondary to hyperglycaemia. 5. A non-dialyzed (>12,000-Mr) material in plasma from diabetic, but not normal animals, decreased the number of rolling, sticking and migrated leukocytes in recipient rats. This effect was completely abolished by chronic treatment of diabetic plasma donors with aminoguanidine. 6. The results suggest that a protein modified by glycosylation (>12 kDa) is associated with leukocyte dysfunction in diabetes mellitus and that the ability of aminoguanidine to prevent such dysfunction is related to an inhibitory effect on advanced glycation end products formation.