A study of the immunosuppressive systems of rats has been conducted with special attention to whether suppressor cells can be induced to down-regulate the efferent limb of contact sensitivity. Contact sensitivity (CS)1 was induced in DA rats 5 days after immunization with trinitrochlorobenzene (TNCB). Intravenous pretreatment of naive rats with TNP-coupled syngeneic spleen cells 7 days before sensitization suppressed the induction of CS by 60%. Suppression of the inductive phase of CS could be transferred adoptively into syngeneic rats with spleen cells of such tolerized animals. Cell fractionation studies showed the OX8+ (CD8) T cell population (cytotoxic/suppressor) was responsible for the suppression in the afferent phase of CS. Such cells were incapable of suppressing preexisting CS. To investigate whether suppression could be induced for the efferent phase, spleen and peritoneal exudate cells (PEC) from rats tolerized by administering TNP-spleen cells iv plus epidermal paintings with TNCB were adoptively transferred into recipients sensitized 4 days earlier. Both spleen cells and PEC suppressed the efferent phase of CS but PEC did so more efficiently. Separation of splenic cells revealed the suppressors to be CD8+ T cells. Furthermore, separation of PEC into plastic adherent and nonadherent cells showed the nonadherent (T cell enriched) cells to be noneffective alone. The adherent subpopulation conveyed suppression but did so more effectively upon addition of the T cells. Thus, T cells and macrophages may operate in concert to achieve suppression of the efferent limb of CS. PEC from tolerized rats suppressed performed CS of any specificity but only after the suppressor cells were triggered with the same antigen that induced them. Since both the afferent and efferent phases of CS have now been shown to be suppressable, two separate suppressor mechanisms may be operable in rats.