Kinetic analysis of residue and outflow curves of gamma-emitting indicators such as chromium-51-EDTA and iodide-131-thalamate from skeletal muscle gives the possibility to determine the extraction fraction and the plasma flow, and from these two values the capillary diffusion capacity can be calculated (Sejrsen 1970, preliminary report). This is possible both for the transport from blood to tissue and from tissue to blood. THis alternative method has been compared in the autoperfused cat gastrocnemius preparation with the indicator diffusion method based on venous registration of a diffusible test indicator and an intravascular reference indicator (Chinard et al. 1955, Crone 1963). The results of the five independent measurements show good agreement. Calculation of the permeability Pd based on a capillary surface area of 7 000 cm2/100 g of tissue gives values of 1.05 . 10(-5), 1.10 . 10(-5), and 1.16 . 10(-5) cm/s, which is in agreement with results obtained by other investigators. The permeability was equal in both directions, and thus the capillary membrane seems to function as a symmetrical membrane. Using an area of 5 000 cm2/100 g which presumably is more realistic at the plasma flow range used gives Pd values around 1.5 . 10(-5) cm/s. The effective pore area is calculated to constitute 1/50 000 of the capillary surface area. Calculation of volumes of distribution in the muscle tissue gave intravascular plasma volumes of 1.5 to 2.0 ml/100 g, an extravascular volume of 12.4 and 15.2 ml/100 g and a final monoexponential component constituting a compartment of 5.4 and 7.0 ml/100 g from residue and venous curves, respectively. The last mentioned compartment constitutes nearly 50 per cent of the extravascular space, and it is suggested, that it is located inside the sarcoplasmic reticulum, which anatomically constitutes about 50 per cent of the interstitial space. The total area of contact between the longitudinal and the transversal tubules in this subsystem, which is the membrane of the lateral saccus, is estimated to about 6 times the capillary surface area at a plasma flow of 15 ml/100 g . min which gives a permeability about 60 times lower for this membrane compared to the capillary membrane.