Diffusion of uniformly dispersed tritium-labeled estradiol in water-swollen poly(ethylene oxide) -based hydrogels was studied by assaying the release of solute from cylindrical hydrogels into a finite volume of solution at 37-degrees-C. Fractional release followed a t0.5 relationship for a range of radii between 0.20 and 0.35 cm and different polymer compositions with equilibrium water uptake of 220-750 parts per hundred dry polymer. Values of the diffusion coefficient were calculated from the fully swollen hydrogels, which represents the swelling and release profile quite accurately.