Airway and pulmonary tissue responses to aerosolized histamine were studied in five mongrel puppies (8-10 wk old). Alveolar pressure was measured by use of alveolar capsules and respiratory mechanics calculated during tidal ventilation and flow interruptions. Aerosolized histamine caused an increase in the tissue viscoelastic properties, which was measured as an increase in pulmonary resistance during tidal ventilation. An increase in stress recovery of the pulmonary tissues was measured with the interrupter technique after aerosolized histamine. These data demonstrate that aerosolized histamine caused an increase in the tissue viscoelastic properties. The most reasonable explanation for the mechanism of this increase would seem to be via reflex pathways stimulated by centrally located receptors.