Increases in the myocardial level of cGMP usually exert negative inotropic effects in the mammalian hearts. We tested the hypothesis that the negative functional effects caused by nitric oxide (NO) or C-type natriuretic peptide (CNP) through cyclic GMP would be blunted in hypertrophied cardiac myocyte. Contractile function, guanylyl cyclase activity, cGMP-dependent protein phosphorylation and calcium transients were assessed in ventricular myocytes from aorta-stenosisinduced hypertrophic and age-matched control mice. Basal percentage shortening was similar in control and hypertrophic myocytes. S-nitroso-N-acetyl-penicillamine (SNAP, nitric oxide donor, 10^{- 6} and 10^-5 M) or CNP (10^-8 and 10^-7 M) reduced percentage shortening (PCS) in both groups, but their effects were blunted in hypertrophic myocytes. Maximal rates of shortening and relaxation were depressed at basal level and both reagents had attenuated effects in hypertrophy. Similar results were also found after treatment with guanylin and carbon monoxide, other stimulators of particulate and soluble guanylyl cyclase, respectively. Guanylyl cyclase activity was not significantly changed in hypertrophy. Addition of Rp-8-[(4-Chlorophenyl)thio]-cGMPS Triethylamine (inhibitor of cGMPdependent protein kinase, 5x10^-6 M) blocked SNAP or CNP's effect in control mice but not in hypertrophy, indicating the cGMP-dependent kinase (PKG) may not mediate the actions of cGMP induced by NO or CNP in hypertrophic state. Calcium transients after SNAP or CNP were not significantly changed in hypertrophy. These results suggest that in hypertrophied mice, diminished effects of NO or CNP on ventricular myocyte contraction are not due to changes in guanylyl cyclase activity. The data also indicated that PKG-mediated pathways were diminished in hypertrophied myocardium, contributing to blunted effects.