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The retrotrapezoid nucleus (RTN) contains central respiratory chemoreceptors that are inhibited by activation of slowly adapting pulmonary stretch receptors (SARs). Here we examine whether RTN inhibition by lung inflation could be mediated by a direct projection from SAR second-order neurons (pump cells). Pump cells (n = 56 neurons, 13 rats) were recorded in the nucleus of solitary tract (NTS) of halothane-anesthetized rats with intact vagus nerves. Pump cells had discharges that coincided with lung inflation as monitored by the tracheal pressure. Their activity increased when end-expiratory pressure was raised and stopped instantly when ventilation was interrupted in expiration. Many pump cells could be antidromically activated from RTN (12/36). Nine of those were labeled with biotinamide. Of these nine cells, eight contained glutamic acid decarboxylase 67 (GAD67) mRNA and seven were found to reside …