Local cerebral blood flow was measured in rats by the 14C-labeled iodoantipyrine technique with quantitative autoradiography during the processing of environmental stimuli. Presentation of a tone increased blood flow in the auditory but not the visual pathway. When the animal had previously been conditioned to fear the tone, blood flow additionally increased in the hypothalamus and amygdala. Local cerebral blood flow can thus be used to detect patterns of cerebral excitation associated with transient (30- to 40-second) mental events in experimental animals.