Local cerebral blood flow (1CBF) was measured simultaneously in ten cats with (1) a large surface thermal diffusion probe resting on the cortex and (2) hydrogen clearance curves from implanted electrodes surrounding the thermal probe. A close correlation was found between 1CBF values obtained by the two methods. Since hydrogen clearance is accepted as quantitative, the data suggest that the thermal diffusion technique is a reliably quantitative means of measuring local cerebral blood flow.