The number of types 1 and 2 synapses on chief cells per unit area in the rat carotid body were compared throughout pre- and postnatal periods. Chief cells are postsynaptic in type 1 synapses and presynaptic in type 2 synapses. Type 2 synapses increased in number rapidly as development proceeded, while type 1 synapses remained few. By the 50th postnatal day the numerical density of synapses of both types reached a plateau, where type 2 synapses were approximately 22 times more numerous than type 1 synapses. The present findings suggest that nerve fibers innervating chief cells of the carotid body are basically sensory.