Isolated semitendinosus muscles of the toad were incubated in both Ringer solution and toad plasma for periods ranging from 45 to 170 minutes. The extracellular volume (ECV), as measured from the equilibrium distribution of 35SO4(2-), was found to increase (in gram extracellular (EC) water/gram wet weight) from 0.15 +/- 0.01 to 0.20 +/- 0.02 in muscles incubated in Ringer solution, whereas the ECV was found to remain constant in tissues incubated in plasma (0.16 +/- 0.01 to 0.17 +/- 0.01), for periods up to 170 minutes. Since the true ECV, as measured morphometrically, was found to remain unchanged for muscles incubated in Ringer solution for up to 3 hours, the data for these muscles were interpreted to represent a change in anion permeability of the cellular membranes in response to the absence of some plasma fraction from the Ringer solution (with an electrolyte composition and pH similar to that of plasma). When toad plasma albumin was added to the Ringer solution, in physiological concentrations, the ECV of the muscles was observed to remain relatively constant during the 3-hour incubation period. This albumin effect on anion permeability was found to be albumin concentration-dependent. Further, other sources of albumin were found to have varying degrees of effect on the anion permeability of toad muscle, with bovine albumin serving as the most acceptable substitute for toad albumin, when incubating semitendinosus muscles.