The respiratory response to acute inhalation of hydrogen sulfide (H2S) and the response of pulmonary CO2 receptors to this gas were studied in male White Leghorn chickens. Inhaling low concentrations of H2S (0.05%) for 30 min had no effect on ventilation, but respiratory frequency and tidal volume became irregular and variable in birds that inhaled 0.2% and 0.3% H2S for that period. All birds that inhaled 0.4% H2S died within 15 min. H2S, presented in the gas stream of unidirectionally ventilated birds, caused an increase in the discharge frequency of intrapulmonary CO2 receptors and an increase in the amplitude of sternal movements. Because an increase in the discharge of these receptors normally inhibits the central respiratory neurons and may lead to apnea, it is clear that H2S also has actions that increase the output from these central neurons. The possibility that H2S affects the intrapulmonary CO2 receptors by inhibiting carbonic anhydrase in them is discussed.