1. The absorption, distribution and elimination of a new alkylating agent, N,N-bis(2'-chloroethyl)4-amino-2-methyl-1-methoxynaphthalene (mitoclomine), labelled with 14C either in the dichloroethyl group or in the methoxy group, have been studied in mice and rats. 2. Expired 14CO2 of rats given [chloroethyl-14C[- or [methoxy-14C]mitoclomine amounted to 1.0 and 35% dose respectively, indicating that O-dealkylation occurs. 3. The part of the molecule carrying the cytotoxic group displayed a high affinity for lymphoid organs (thymus, spleen). This could explain some biological properties of this drug, especially the selective effects of mitoclomine on circulating lymphocytes.