Protein kinase C (PKC)-induced phosphorylation of cardiac troponin I (cTnI) depresses the acto-myosin interaction and may be important during the progression of heart failure. Although both PKCbetaII and PKCepsilon can phosphorylate cTnI, only PKCbeta expression and activity are elevated in failing human myocardium during end-stage heart failure. Furthermore, although increased cTnI phosphorylation was observed in mice with cardiac-specific PKCbeta II overexpression, no differences were observed in cTnI phosphorylation status between wild type and cardiac-specific PKCepsilon overexpression mice. A potentially important downstream effector of PKCs is p90 ribosomal S6 kinase (p90RSK), which plays an important role in cell growth by activating several transcription factors as well as Na+/H+ exchanger. Since both Ser23 and Ser24 of cTnI are contained in putative consensus sequences of p90RSK phosphorylation sites, we hypothesized that p90RSK is downstream from PKCbeta II and can be a cTnI (Ser(23/24)) kinase. p90RSK, but not ERK1/2 activation, was increased in PKCbetaII overexpression mice but not in PKCepsilon overexpression mice. p90RSK could phosphorylate cTnI in vitro with high substrate affinity but not cardiac troponin T (cTnT). To confirm the role of p90RSK in cTnI phosphorylation in vivo, we generated adenovirus containing a dominant negative form of p90RSK (Ad-DN-p90RSK). We found that the inhibition of p90RSK prevented H2O2-mediated cTnI (Ser(23/24)) phosphorylation but not ERK1/2 and PKCalpha/betaII activation. Next, we generated cardiac-specific p90RSK transgenic mice and observed that cTnI (Ser(23/24)) phosphorylation was significantly increased. LY333,531, a specific PKCbeta inhibitor, inhibited both p90RSK and cTnI (Ser(23/24)) phosphorylation by H2O2. Taken together, our data support a new redox-sensitive mechanism regulating cTnI phosphorylation in cardiomyocytes.