In this paper experimental results on electron spin dynamics in a 3D Heisenberg ferromagnet are presented. These experimental results, the spectral densities phi ₀⁺( omega ), phi ₀^z( omega ) and phi ₀₁⁺( omega ) of the spin auto- and pair correlation functions, have been measured in Rb₂CuBr₄.2H₂O at relatively high temperatures (T=20K, 77K, so that kT>>JS²+g mu _BB). Comparison of these results with some current theoretical models on spin correlations shows a good high-frequency/short-time agreement. The low-frequency part/diffusion region, shows a remarkable discrepancy, which can be explained by a too high theoretical diffusion constant. The authors' experimental diffusion constant is determined to be D_exp=0.31+or-0.03 J₁a²/h(cross).