Passive transport of macromolecules in growing nerve processes was analyzed quantitatively by measuring the rate of diffusion of fluorescently labeled molecules injected into the soma of cultured Xenopus neurons. We found that the diffusion of globular proteins in the neurite's cytoplasm was about five times slower than that in aqueous solution, a rate considerably higher than those inferred from previous studies on cultured non-neuronal cells. The dependence of the diffusion coefficient, D, on the size of diffusing molecules was examined by measuring the diffusional spread of fluorescently labeled dextrans over a wide range of molecular weights. We found that the size dependence of D deviates considerably from that expected for diffusion in a viscous aqueous medium: larger dextrans encounter disproportionately higher viscous resistance. Treatment of the neuron with the microfilament-disrupting agent cytochalasin B, or pre-loading of the cells with dephospho-synapsin I, a molecule that induces bundling of actin filaments, significantly increased the diffusion rate for large dextrans without affecting that of small dextrans. Taken together, these results provide a quantitative basis for assessing diffusion as a potential transport mechanism along nerve processes, and suggest that the microfilament meshwork imposes a selective constraint on the diffusion of large macromolecular components within the neuronal cytoplasm.