The fluorescence recovery after photobleaching technique (FRAP) was used to measure the diffusion coefficients (D) of fluorescein isothiocyanate (FITC)-dextrans in diluted, semidiluted, and concentrated hyaluronic acid solutions. The decrease of the diffusion coefficients as the hyaluronic acid concentration increases was consistent with the universal scaling equation D/D-0 = exp(-ac(v)). The diffusion experiments were carried out to obtain structural information on the transient network structure in hyaluronic acid solutions. On the basis of scaling laws, the concentration dependence of the average mesh size xi was determined as xi similar to c(-0.58+/-0.07). Additionally, the average xi-values were estimated. The concentration dependence of xi and the absolute vlaues for xi were compared with structural information obtained from theological experiments performed on hyaluronic acid solutions. Even though accurate results were found for the concentration dependence of xi, there was a semiquantitative relation between the xi-values estimated from the diffusion experiments and the theological experiments.