1. The isotopic water permeability coefficient, P, of the plasma membrane of frog body cavity eggs has been determined as a function of the incubation time in media of different tonicity.
2. It was found that the permeability decreases with the incubation time in hypotonic solutions. The observed changes may be correlated with an increase of the tension in the vitelline membrane.
3. In the evaluation of the experimental results the diffusion of water in the external medium (‘unstirred layers’) is taken into account.