(1) Glass microelectrodes were used to record extracellularly the activity of single dorsal (in the region of nucleus tractus solitarius (NTS)) inspiratory neurons in either chloralose- or pentobarbitone-anaesthetized, paralyzed cats. (2) In the lateral and ventral part of NTS, 89 cells were localized which responded antidromically to stimulation applied to the contralateral spinal cord (C3). Seventy were not excited by lung inflation and were classified as Ralpha, while 19 excited by that stimulus were classified as Rbeta. Usually Rbeta neurons fired both during each pump-induced lung inflation and during the central inspiratory excitation (CIE). (3) A technique similar to Merrill's recording small variations in the antidromic latency measured to the SD spike was used to ascertain excitability of Ralpha and Rbeta cells during the CIE time and during lung inflation in expiration. The technique was extended by averaging antidromic latency patterns within many respiratory cycles. (4) In the majority of Ralpha cells the averaging revealed a shortening of the antidromic latency during lung inflation in expiration, which indicated a subthreshold depolarization. Lengthening of the antidromic latency by this stimulus was never observed. These results are in disagreement with the concept of a direct inhibitory input from Rbeta to Ralpha neurons. (5) It is concluded that the dorsal Ralpha and Rbeta neurons differ less in their properties than was previously assumed on the basis of classical extracellular techniques. The inhibitory influence of pulmonary stretch receptor input on inspiratory discharge of Ralpha cells seems to be exerted through an indirect action on the source of the CIE.