Mediation of the respiratory reflex effects of an exogenous serotonin challenge goes beyond the lung vagi and is suggested to involve the nodose ganglia. In the present experiments the effects of an intravenous serotonin challenge on breathing pattern were studied in 8 pentobarbitone-chloralose anaesthetised cats. Bolus injection of serotonin oxalate (50 µg/kg) into the right femoral vein evoked prompt apnoea of 19.2 (± 2.4)-second duration in all 8 cats while intact; the apnoea was much shorter after midcervical vagal section (8.1 ± 0.9 s, p < 0.001), and was abolished by supranodose vagotomy. In post-apnoeic breaths, the tidal volume was reduced from a baseline of 34.1 ± 4.0 to 13.5 ± 1.1 ml (p < 0.001) prior to, and from a baseline of 43.9 ± 5.4 to 33.8 ± 6.6 ml (p < 0.01) after midcervical vagotomy; the serotonin challenge did not affect tidal volume following supranodose vagal section (p = 0.4). The increase in respiratory rate found in intact (p < 0.001) and midcervically vagotomised cats (p < 0.01) was eliminated by the neurotomy above the nodose ganglia. Supranodose vagotomy altered cardiovascular response to serotonin by replacing the fall in blood pressure with an increase. These data suggest that the sequelae of serotonin-induced pulmonary chemoreflex, i.e. respiratory arrest, cardiovascular changes and post-apnoeic pattern of breathing require intact nodose ganglia.