The Peltier stack, which allows quantitative measurement of cerebral blood flow (CBF) by means of thermal diffusion, has a probe too large for use in small animals. However, it is difficult to measure CBF by a thermal diffusion method involving the use of a constantan heating wire, because of heat conduction between the two gold plates. The authors developed a new thermal diffusion flow probe, using a constantan wire as a heat source rather than the Peltier stack. With the new probe, separation of the gold plates and attachment of a pair of long thermocouples minimize heat conduction between the two plates. Moreover, the probe itself is considerably smaller than that of the Peltier stack. The new probe was inserted to the subdural space of rabbits and the voltage (V; mV) was measured with an amplifier by the circuit of constant current method. CBF (F; ml/100 g/min) was measured simultaneously by the hydrogen clearance method in the adjacent cortex. A regression equation of F = 29, 111 (1/V-1/226) was obtained between 35 pairs of F and V (r = 0.92, p less than 0.001), which verified that CBF can be accurately measured with the new probe.