We investigated the effects of autologous bone marrow stem cell transplantation in a rat model of Huntington's Disease. Thirteen days after bilateral quinolinic lesions (QA), bone marrow was implanted into the damaged striatum. The ability of the transplants to reverse QA-induced cognitive deficits in the radial-arm water maze (RAWM) was examined. The transplants significantly reduced working memory deficits. Most of the transplanted cells appeared quite primitive. Because only a few cells expressed neural phenotypes, we suggest that the release of growth factors by the transplants allowed surviving cells within the caudate to function more efficiently and to facilitate other compensatory responses.