1. The activity of pump (p.) cells, second-order neurones in the pulmonary stretch receptor pathway, was recorded extracellularly in the nucleus of the tractus solitarius (n.t.s.) of the decerebrate cat. Their firing was proportional to changes in lung volume but unrelated to the centrally determined respiratory rhythm. A systematic search of the n.t.s. for the location of p. cells was made and an assessment of their efferent projection to the contralateral n.t.s. was determined electrophysiologically by the antidromic mapping technique. 2. P. cells were located around, and in close proximity to, the solitary tract. The two sites of greatest density were ventromedial and dorsolateral to the tract, with lower concentrations found laterally and ventrolaterally. 3. For twelve of the thirty p. cells tested, evidence of a projection to the contralateral n.t.s. was obtained; in seven of these cells, axonal arborizations within the projection area were identified. Almost all the cells that sent axons to the contralateral n.t.s. were located dorsolateral to the tract; there was no evidence that cells in the ventromedial region had contralateral projections. 4. No evidence that R beta neurones project to the contralateral commissural and ventrolateral subnuclei was found. 5. No p. cells projected to the contralateral ventrolateral n.t.s. The site of projection and branching was consistently localized just caudal to the obex and medial to the solitary tract, in the caudal medial, and commissural subnuclei of the n.t.s. This same region has been shown to receive a dense, direct projection from pulmonary rapidly adapting receptors.