The heterogeneity of blood flow in the brain under normo- and pathophysiological conditions, as well as during functional activation, has stimulated an interest in the use of autoradiography as a technique for the measurement of local cerebral blood flow. [¹⁴C ]iodoantipyrine is the most prevalent tracer for the autoradiographic measurement of local cerebral blood flow since it is inert, nonvolatile, and is readily diffusible through the blood–brain barrier. The ability to diffuse freely in cerebral tissue, however, can lead to significant errors if the time duration between when the animal is sacrificed and when the tissue is frozen becomes appreciable, leading to significant postmortem diffusion of the tracer. Using an in vitro technique, the bulk diffusion coefficient for [¹⁴C ]iodoantipyrine was measured in brain tissue (2.1×10⁻⁶ cm²/s). Cerebral blood flow was measured with [¹⁴C ]iodoantipyrine in anesthetized rats. At the end of the radiotracer infusion, the brain was freeze-captured using a device consisting of two rapidly spinning stainless steel blades that were pneumatically driven through the head, freezing the tissue several hundred milliseconds following sacrifice. Autoradiograms from these brains exhibit considerable heterogeneity in blood flow. Computer simulations of the effect of tracer diffusion on these autoradiograms show significant degradation of the images highlighting the importance of very rapid postmortem freezing.