[en] The thin layer sectioning technique has been used in order to determine the solute diffusivity of ⁵⁹Fe tracers in the light rare-earth metals cerium, praseodymium, and neodymium. The measurements were made both in the close-packed (fcc for Ce and double-hcp for Pr and Nd) and in the high-temperature bcc phases of the solvent metals. The diffusivities of iron are extremely high, with values exceeding 10^-5 cm²/sec at temperatures above 0.7 T/sub m/ (T/sub m/ melting temperature of the solvent metal). The activation energies are much lower than the activation energy for self-diffusion, of the order of 10 kcal/mole. The results indicate that iron is a fast-diffusing solute, its diffusion in the rare-earth metals taking place probably by means of the dissociative mechanism, with a predominantly interstitial component. Several simultaneous measurements of the diffusivity of ⁵⁴Mn suggest that Mn is also a fast-diffusing solute, though to a lesser extent than ⁵⁹Fe. (U.S.)