Thyroid hormone (TH) regulation of cellular proliferation was demonstrated, but their effect and involved mechanisms on lymphocyte activity has not been elucidated. Also, differential expression of protein kinase C (PKC) isoenzymes and high nitric oxide synthase (NOS) activity were described on tumor respect to normal T lymphocytes. Here direct actions that TH exert on normal T lymphocytes and BW5147 T lymphoma cells related to PKC and NOS activities were analyzed. TH hormones increased tumor and mitogen-induced normal T lymphocyte proliferation. These effects were impaired on both cell types by PKC isoenzyme selective blockers, indicating the participation of Ca²⁺-dependent or independent isoenzymes on normal and tumor cells respectively. TH actions were blunted by extra- and intracellular Ca²⁺ blockers only on normal T lymphocytes, while NOS blockers impaired TH-induced proliferation on T lymphoma cells. Twenty-four hours incubation with TH induced a rise in total and membrane-associated PKC activities on both cell types and lead to a rapid and transient effect only on tumor cells. TH increased atypical PKC expression on BW5147, but classical PKC isoenzymes on mitogen-stimulated normal T cells. Additionally, TH augmented NOS activity and inducible NOS (iNOS) protein and gene expression only on tumor cells. Blockade of PKC and of the atypical PKC isoform, inhibited TH-mediated stimulation of both iNOS and cellular proliferation. These results show, for the first time, differential intracellular signals involved in TH modulation of lymphocyte physiology and pathophysiology.