Action potentials of single vagal afferent nerve fibers from slowly adapting pulmonary stretch receptors, transpulmonary pressure and rate of air flow were recorded in anesthetized cats with open thoraxes during varied volumes and rates of inflation and deflation of the lungs. Both discharge frequency and total transpulmonary pressure varied with lung volume and also with its rate and sign of change. Effects of acceleration of volume on discharge frequency were not observed. The receptors provide signals related to the work of breathing.