Background and objective: Nitric oxide (NO) and related nitrovasodilators regulate blood pressure by activation of soluble guanylate cyclase, elevation of cyclic guanosine monophosphate (cGMP), and activation of cGMP-dependent protein kinase (cGPK). Despite the progress of our understanding of the NO/cGMP mediated vasorelaxation, partly through conventional cGPK knock-out mice, the role of cGPK remains unclear. In particular, the downstream target(s) of the kinase are not well defined. We hypothesized that highly selective inhibitors of cGPK delivered in vivo in adult may elucidate the role of the kinase in vasorelaxation and regulation of blood pressure. Methods and results: We have adopted a newly developed method of TAT-mediated protein transduction to study NO/cGMP signaling pathways in mice. In vitro, TAT-cGPK inhibitor peptide blocked autophosphorylation of the kinase. The effect of cGPK inhibition on murine blood pressure (BP) was investigated by continuous infusion of 100 μg of the inhibitor into the internal jugular vein over 72 hours. In 8 animals infused with the inhibitor, the mean BP increased by 38 ± 24/31 ± 30 mm Hg (from 108 ± 14/92 ± 19 to 145 ± 13/123 ± 19 mm Hg) whereas in 8 animals injected with either saline (4) or TAT-green fluorescent protein (4), the BP remained the same (from 117 ± 21/101 ± 26 to 119 ± 22/96 ± 30 mm Hg); P = 0.001. Ex vivo, using vascular ring assays, NO-dependent relaxation in murine aortas harvested from animals administered with TAT-cGPK inhibitor was inhibited by 25% (sham 76 ± 11%, inhibitor 51 ± 13%). Conclusion: We demonstrated that highly specific peptide inhibitor of cGPK induced adult murine hypertension through inhibition of nitric oxide mediated relaxation. © 2012 Elsevier Ireland Ltd.