We measured surface tension in individual alveoli by observing the spreading properties of fluid droplets placed by micropipette on the alveolar surfaces. The test fluids were calibrated on monolayers of dipalmitoyl phosphatidylcholine spread at the air-saline interface of a captive bubble. The air bubble was floated by buoyancy against a ceiling of 0.5% agar. The bubble surface tension could be altered by inflating or deflating the bubble, and the value of the surface tension was determined by shape analysis for the sessile drop. Test fluid droplets were placed by micropipette onto the upper, flat bubble surface and the diameters of these droplets were measured with a microscope. In cat lungs held at 40% total lung capacity and 37 degrees C the surface tension remained below 1 mN . m -1 for about 10 min, and then increased slowly in a linear fashion to 9 mN . m -1 in 70 min. During stepwise deflation from 70% to 40% total lung capacity the surface tension changed from approximately 10 mN . m -1 to less than 1 mN . m -1. At each step during deflation we compared surface tension in alveoli of differing size and location. At any given lung volume in the range between 70% and 40% total lung capacity we found equal values for the alveolar surface tension regardless of alveolar size and location.