The nude trait in the rat is transmitted in an autosomal recessive manner and is associated with thymic aplasia, T-cell deficiency, and hairlessness. Congenic rats homozygous for the R N U (Rowett nude) locus are important models in the study of inflammatory disease, tumor growth, and transplant rejection. The R N U locus has not been previously mapped, and the nature of the gene product is unknown. To determine the map location of this gene, a single F344.rnu/rnu (athymic nude congenic Fischer rat) male congenic rat was bred with 3 LEW/N (NIH stock Lewis rat) female rats to produce F 1 progeny. Twelve F1 brother-sister breeding pairs were established. Forty-nine phenotypically nude F 2 offspring (198 total) were obtained. Linkage analysis done on F 2 DNA revealed highly significant cosegregation between the nude phenotype and eight polymorphic markers located on Chromosome (Chr) 10. The tightest linkages were with: MYH3 (embryonic, skeletal myosin heavy chain) and SHBG (sex hormone-binding globulin), giving 2 point lod scores of 20.2, and 20.0, respectively. The map order and map distances, determined by multipoint linkage calculations, were: RR24-(16.1 cM)-MYH3-(3.5 cM)-SHBG-(4.7 cM)-R N U -( l l . 9 cM)-F16F2-(24.1 cM)-CLATP (citrate lyase ATPase)-(2.4 cM)-ACE (angiotensin converting enzyme)/PPY (pancreatic polypeptide)-(14.1 cM)-RR1023. The position of the R N U locus in the rat corresponds closely with that of the recently reported nu locus in the mouse. This finding suggests that the nude phenotype in the rat and the mouse arise from defects in homologous genes.