Evening primrose oil (EPO) is rich in the omega-6 essential fatty acid component, gamma-linolenic acid. The aim of the investigation was to determine whether EPO treatment prevented a reduction in sciatic nerve perfusion and oxygenation in streptozotocin-diabetic rats. Rats were treated from diabetes induction with 10 g EPO kg-1 day-1. Sciatic blood flow was measured by microelectrode polarography and hydrogen clearance. Diabetes caused 47.7% +/- 3.4% (P < 0.001) and 58.8% +/- 4.8% (P < 0.001) reduction in the nutritive (capillary) and the non-nutritive (large vessel) components of endoneurial blood flow, respectively, which were prevented by EPO. Treatment had no significant effect on nutritive flow in non-diabetic rats; however, the rate of non-nutritive flow increased by 97.7% +/- 38.9% (P < 0.01). Sciatic endoneurial oxygen tension was measured by microelectrode polarography. Diabetes resulted in a 44.7% +/- 3.4% reduction in mean oxygen tension (P < 0.001), which was largely (82.3% +/- 10.2%) prevented by EPO treatment (P < 0.001). Thus, EPO prevents impairment of blood flow and endoneurial oxygenation in experimental diabetes. It is likely that this neurovascular action accounts for the beneficial effects of treatment on nerve function in diabetic rats and patients.