Bulk shear viscosities were measured with a cone and plate microviscometer as a function of concentration, shear rate, and temperature for lavaged calf lung surfactant (LS), Exosurf, Infasurf, Survanta, and synthetic lipid mixtures dispersed in normal saline. Viscosity increased with phospholipid concentration for all surfactants, but its magnitude and shear dependence varied widely among the different preparations. Saline dispersions of Exosurf and synthetic phospholipids had low viscosities of only a few centipoise (cp) and exhibited minimal shear dependence. LS, Infasurf, Survanta, and lipid mixtures containing palmitic acid and tripalmitin had larger non-Newtonian viscosities that increased as shear rate decreased. At 35 mg of phospholipid/ml and 37°C, viscosity values were 52.3 cp (Survanta), 31.1 cp (LS), and 25 cp (Infasurf) at a shear rate of 77 s¹ and 16.9 cp (Survanta), 10.1 cp (LS), and 6.6 cp (Infasurf) at 770 s¹. At 25 mg of phospholipid/ml and 37°C, viscosity values at 77 s¹ were 28.8 cp (Survanta), 4.7 cp (LS), and 12.5 cp (Infasurf). At fixed shear rate, viscosity was substantially decreased at 23°C compared with 37°C for LS and Infasurf but was increased for Survanta. Calcium (5 mM) greatly reduced the viscosity of both Survanta and Infasurf at 37°C. Studies on synthetic mixtures indicated that phospholipid/apoprotein interactions were important in the rheology of lung-derived surfactants and that palmitic acid and tripalmitin contributed to the increased viscosity of Survanta. The viscous behavior of clinical exogenous surfactants potentially influences their delivery and distribution in lungs and varies significantly with composition, concentration, temperature, ionic environment, and physical formulation.