Recent studies indicate that blood flow to cerebral cortex is not homogeneous but may vary both spatially and temporally. In addition, some investigators have reported that capillaries and arterioles can be recruited to increase cerebral blood flow, an issue that is extremely controversial. The goal of this study was to determine whether recruitment of cerebral blood vessels is an important mechanism in spatial and temporal heterogeneity of cerebral blood flow. In seven anesthetized ventilated rats, different fluorescent tracers were injected 45 and 10 s before decapitation. In addition, [14C]iodoantipyrine also was injected 10 s before decapitation. After the brains were sectioned, fields in the cerebral cortex were examined microscopically for fluorescence and processed for measurement of cerebral blood flow with techniques of quantitative autoradiography and image analysis. With examination of 24 +/- 2 (SE) points in cerebral cortex of each rat, similar numbers of small blood vessels (< or = 10 microns) were counted that contained fluorescent tracers injected 45 and 10 s before decapitation (346 +/- 48 and 355 +/- 42 vessels/mm2, respectively; P > 0.05). Large blood vessels (20-60 microns; 73 +/- 6 vessels in each rat) contained both fluorescent tracers. In addition, adjacent regions of high and low blood flow contained similar numbers of small and large vessels. Our findings indicate that vascular recruitment is not an important mechanism in temporal or spatial heterogeneity of cerebral blood flow.