Graded inflation and deflation pressures were transiently applied to the airway, and the effects on the inspiratory (TI) and expiratory (TE) durations were studied in twelve Japanese white rabbits tracheotomized and anesthetized with urethane and chloralose. Without vagal cooling, lung inflation prolonged TE, whereas changes in TI were small and inconsistent. The bilateral vagus nerves were cooled to 7 degrees C to block myelinated fibers, and to study the effects of C-fiber afferents. During vagal cooling to 7 degrees C, inflation dose dependently prolonged the first TI with no consistent change in TE. Without vagal cooling, lung deflation shortened TE of the first and succeeding breaths, and prolonged TI of the first breath to manifest apneusis. Deflation applied at the vagal temperature of 7 degrees C induced slight shortening of TI and TE in 6 of the 12 rabbits. All these changes were blocked when the vagus nerves were cooled to 1 degree C. The results may indicate that vagal afferents from irritant receptors shorten TE, and C-fiber afferents prolong TI. High irritant afferent activity may markedly shorten TE, causing tidal inspiratory activities to merge and manifest apneusis. Capsaicin, which is suggested to be a "selective" stimulant of C-fiber receptors was injected into the right atrium in 4 of the 12 rabbits. The chief changes were TE shortening without vagal cooling, and TI prolongation during cooling to 7 degrees C. This suggests that capsaicin injected into the pulmonary circulation stimulates both the irritant and C-fiber receptors and the C-fiber afferents may prolong TI even when stimulated chemically.