Abstract The activity of respiratory neurons in the caudal part of the nucleus retroambigualis (NRA) was recorded intracellularlyin decerebrated, spontaneously breathing cats. Spinal projections of these neurons were determined by antidromic stimulation at the C 3 segment. A high concentration of bulbospinal expiratory (BS-E) neurons was identified in the caudal NRA, whereas the inspiratory (I) neurons, not activated antidromically, were also found to be intermingled in the same region. The BS-E neurons had ramp-like depolarizing potentials during expiration, and repolarized rapidly at the onset of phrenic nerve discharge. The I neurons depolarized abruptly in the early I phase, and repolarized gradually thereafter, namely, they were early-I neurons. Intracellular current injections revealed postsynaptic inhibition of the BS-E neurons during inspiration, as evidenced by inhibitory postsynaptic potential reversal. Using the technique of intracellular labeling with horseradish peroxidase, seven well-stained expiratory cells located in the caudal NRA revealed detailed information about axonal morphology: the axon projected rostrally and dorsomedially for the first 2 mm after emerging from the soma, then turned caudally and ventrally along two different courses, and crossed the midline of the medulla almost at the same rostrocaudal level as the soma. No axon collaterals were observed along the length of the stained portion, indicating that the BS-E neurons cannot influence other respiratory neurons in the brainstem. It has been concluded that the NRA expiratory neurons are involved only in spinal action, and that they receive a postsynaptic inhibition during inspiration. The neighboring early-I neurons are likely to be candidates for the inhibitory inputs to the BS-E neurons.