A method is described for using measurements of nuclear magnetic resonance (NMR) relaxation times in hydrogels to obtain pore radius distribution profiles in a rapid and noninvasive fashion. Data for spin-lattice relaxation times of the water in the gel are interpreted by using the "magnetization-diffusion" equation that has been used previously for similar measurements in porous glass, sandstones and wood pulp. To account for the fibrous microstructure of hydrogels, a fiber-cell model is introduced and used to demonstrate the existence of a "fast-diffusion" limit, in which NMR relaxation is exponential with a single time constant. It is demonstrated that agar, agarose, and polyacrylamide gels easily satisfy the criterion for fast diffusion, and the model is used to calculate pore-size distributions for those materials. This technique will prove very valuable for real-time, noninvasive monitoring of variations in microstructure occurring over length scales of approximately 100 mu m in both synthetic gels and many foods.