Deep inflation of the lung stimulates surfactant secretion by unknown mechanisms. The hypothesis that mechanical distension directly stimulates type II cells to secrete surfactant was tested by stretching type II cells cultured on silastic membranes. The intracellular Ca²⁺ concentration was measured in single cells, before and after stretching. A single stretch of alveolar type II cells caused a transient (less than 60 seconds) increase in cytosolic Ca²⁺ followed by a sustained (15 to 30 minutes) stimulation of surfactant secretion. Both Ca²⁺ mobilization and exocytosis exhibited dose-dependence to the magnitude of the stretch-stimulus. Thus, mechanical factors can trigger complex cellular events in nonneuron, nonmuscle cells and may be involved in regulating normal lung functions.