Manifestations of the human autoimmune disease systemic lupus erythematosus (SLE) include a number of behavioral and cognitive deficits. The present study asks whether neurobehavioral dysfunction is present also in MRL mice that spontaneously develop most of the fundamental immunological aberrations of SLE. There are two congenic substrains of MRL mice that differ in the time of disease onset: MRL-lpr mice develop lupus early and MRL(-)+/+ develop the typical signs of disease relatively late in life. The behavior of these substrains was assessed at 7 to 11 weeks of age, a time that coincides with the onset of disease in MRL-lpr mice and the absence of known lupus symptoms in the MRL(-)+/+ group. When compared to the congenic MRL(-)+/+ control substrain, MRL-lpr mice were spontaneously less active, traversed a crossbeam slower, and ceased responding to the novelty of a new environment sooner. They were also more reluctant to leave their home base or travel far away from it and perseverated in their response bias during extinction and reversal learning. Immunological status was characterized by moderate proteinuria in both substrains and high titers of antinuclear antibodies in MRL-lpr but not MRL(-)+/+ mice. Histological analysis revealed minimal or no signs of joint pathology in MRL-lpr mice. Thus, this study shows the presence of behavioral dysfunction in mice with early stages of autoimmune disease and gives support for the idea that MRL mice may provide a useful model of neurobehavioral dysfunction in SLE. It is suggested that the behavioral profile of MRL-lpr mice may indicate increased "timidity," related to genetics, autoimmunity, or both.