Krogh's diffusion constant (K) was determined for various inert gases in isolated rat abdominal muscle at 37 degrees C by measuring the amount of gas diffusing per unit time and partial pressure difference through a portion of the muscle of known surface area and thickness. The following mean values for K, in 10(-9) mmol-min-1-cm-1-torr-1, were obtained: C2H2, 42.2; N2O, 20.0; CHClF2, 18.8; H2, 1.67; He, 1.42; CH4, 1.27; SF6, 0.081. From Krogh's diffusion constant, the diffusion coefficient (D) was calculated using the solubility coefficient determined previously in the same preparation. The D values thus obtained were found to be about half the D values in water at 37 degrees C. Model calculations show that for gases with high lipid/water partition coefficient, D in tissues containing lipid is underestimated by this method. Graham's law (inverse proportionality between D and square root of molecular mass) was found to represent a useful approximation for these gases. A better correlation, however, was obtained between D and the molecular diameter.