Murine T cells were stimulated in vivo by administering allogeneic cells or mitogens into the foot pads and then examining the draining popliteal lymph nodes. Allogeneic spleen cells induced the expression of IL2 and IFN-gamma mRNAs in a time- and dose-dependent manner. Induction of these transcripts also was detected after administration of Con A and anti-CD3 mAb. An increase in DNA-synthesizing cells was observed by 48 h, and these were shown to be T cells because of their sensitivity to anti-Thy-1 but not anti-B220 mAb and complement, and because of their localization to the T-dependent areas of the lymph node. The in vivo administration of cyclosporin A (CSA) reduced several of these T cell responses. The level of DNA synthesis and the frequency of cells synthesizing DNA were decreased by approximately 75%, while the induction of IL-2 responsiveness was not substantially diminished. IL-2 and IFN-gamma transcripts were inhibited at least 70-90%, as determined by Northern blot and in situ hybridization. Although the inhibition by CSA was not as complete in animals as observed previously in tissue culture, our findings indicate that in both systems, a major site of action of CSA is to inhibit T cell growth by inhibiting lymphokine production.