The regional density of perfused cerebral capillaries (rDPC) and regional cerebral blood flow (rCBF) were measured in 12 selected brain regions in rats after 3 and 20 weeks of streptozotocin-induced diabetes and in control groups. After 3 weeks of diabetes, both rCBF and rDPC were unchanged in the diabetic group compared to the control group. A diabetes duration of 20 weeks causing bilateral cataracts induced a significant (p less than 0.05) reduction in rCBF in two structures in the visual system compared to the control group (visual cortex: 105 versus 129 ml 100 g-1 min-1; lateral geniculate body: 106 versus 128 ml 100 g-1 min-1) and in the pontine reticular nucleus (82 versus 128 ml 100 g min-1), whereas rDPC remained unchanged. A highly significant correlation between rCBF and rDPC was found in both control groups (r = 0.8, p less than 0.005) whereas the correlation was more scattered in the diabetic groups (r = 0.6, p less than 0.05). The present results show that during chronic diabetes, a reduction of rCBF does not affect the number of perfused capillaries.