The mechanism and cellular targets of mononuclear cell depletion were investigated in strains of mice susceptible or resistant to lethal infection with a virulent street rabies virus (SRV). Significant depletion was evident in the thymus of all infected animals at approximately 5 days postinfection and subsequently involved the spleen and lymph nodes in mice developing clinical signs of rabies. Immunofluorescent analyses of lymphocyte subsets in depleted spleens revealed that cell losses were non-selective since the relative proportions of K+, Thy-1+, Lyt-1+, and Lyt-2+ cells remained unchanged. Diminished expression of I-A membrane glycoproteins on spleen lymphocytes was noted, however, perhaps reflecting reduced availability of I-A-inducing lymphokines. Adrenal hormone toxicity was identified as the cause of mononuclear cell depletion in that mice adrenalectomized before SRV infection showed no evidence of lymphoid depletion. The failure of adrenalectomy to alter anti-rabies antibody responses or SRV lethality also indicates that involution of the lymphoid system is a consequence and not a cause of genetically controlled host susceptibility to SRV. The mechanism of adrenal gland stimulation in rabies-infected mice appears to involve a virus-induced dysfunction in the pituitary gland rather than a stress response to paralysis-induced starvation, based on results of kinetic studies on weight loss, appetite depression, and paralysis in these animals and previous reports of pituitary infection during rabies disease. The relationship of these observations to current theories on rabies virus pathogenicity is discussed.