Elevated protein kinase C activity has been linked to the vascular and neural complications of diabetes. The aim of the present study was to examine the involvement of the β-isoform of protein kinase C in abnormalities of neuronal function, neural tissue perfusion and endothelium-dependent vasodilation in diabetes, by treatment with the selective inhibitor LY333531 (10mg·kg -1 ·day -1 ). Diabetes was induced in rats by streptozotocin; the duration of diabetes was 8 weeks. Nerve conduction velocity was monitored, and responses to noxious mechanical and thermal stimuli were estimated by the Randall–Sellito and Hargreaves tests respectively. Sciatic nerve and superior cervical ganglion blood flow were measured by microelectrode polarography and hydrogen clearance. Vascular responses were examined using the in vitro mesenteric bed preparation. An 8-week period of diabetes caused deficits in sciatic motor (20%) and saphenous nerve sensory (16%) conduction velocity, which were reversed by LY333531. Diabetic rats had mechanical and thermal hyperalgesia. LY333531 treatment did not affect mechanical thresholds, but corrected thermal hyperalgesia. Sciatic nerve and superior cervical ganglion blood flow were both reduced by 50% by diabetes; this was almost completely corrected by 2 weeks of LY333531 treatment. Diabetes caused a 32% reduction in vasodilation of the mesenteric vascular bed in response to acetylcholine, mediated by nitric oxide and endothelium-derived hyperpolarizing factor. When the former was abolished during nitric oxide synthase inhibition, an 80% diabetic deficit in the remaining relaxation was noted. LY333531 treatment attenuated the development of these defects by 64% and 53% respectively. Thus protein kinase Cβ contributes to the neural and vascular complications of experimental diabetes; LY333531 is a candidate for further study in clinical trials of diabetic neuropathy and vasculopathy.