The properties of chemoreceptor and baroreceptor neurones in the petrosal ganglion of the cat were examined in vitro with intracellular micro-electrodes. Chemoreceptor neurones with myelinated axons (average conduction velocity, 11 m/s) showed action potentials with a hump on the falling phase, followed by a prolonged after-hyperpolarization (average duration, 260 ms). The duration of the hump present in the action potential of chemoreceptor neurones was positively correlated with the duration of the after-hyperpolarization. In response to prolonged depolarization, chemoreceptor neurones showed only one or a few action potentials at the beginning of the depolarization. Two types of baroreceptors neurones with myelinated axons were found: fast (F) baroreceptors (average conduction velocity, 33 m/s) and slow (S) baroreceptors (average conduction velocity, 10 m/s). F baroreceptors had action potentials without a hump followed by a short after-hyperpolarization (average duration, 43 ms), while S baroreceptors had spikes similar to those found in chemoreceptors except for a shorter hyperpolarization (average duration, 145 ms). Both types of baroreceptor neurones fired repetitively throughout prolonged depolarization. It is concluded that, in the petrosal ganglion, primary sensory neurones originating a given type of sensory terminal share a particular set of electrophysiological properties.