IN 1953 an athymic hairless mutation was observed in a colony of outbred hooded rats maintained at the Rowett Research Institute. A breeding colony of mutant animals was maintained with difficulty until the early 1960s, when it died out. However, the recessive mutant gene (to which the symbol rnu for Rowett nude has been assigned) was apparently maintained at low gene frequency within the random bred colony. Homozygous mutants were recovered in 1975, and a breeding colony was again established, first in conventional, and later in germ-free conditions, where good breeding performance and survival were obtained. In February 1977 a breeding nucleus of these rats was established at the MRC Laboratory Animals Centre in conventional, germ-free and specific pathogen-free conditions. The observations and data reported here were obtained with small numbers of animals bred in clean, conventional conditions, and suggest the potential usefulness of nude rats in biomedical research.