We tested the hypothesis that the importance of the negative functional effects of the cyclic GMP protein kinase would be reduced in stunned (simulated ischemia/reperfusion) cardiac myocytes. Ventricular cardiac myocytes were isolated from New Zealand white rabbits ( N=7). Myocytes were studied at baseline and after simulated ischemia (15 min of 95% N 2–5% CO 2 at 37 °C) followed by simulated reperfusion (reoxygenation). Cell shortening was studied with a video edge detector; O 2 consumption was measured using O 2 electrodes. Protein phosphorylation was measured autoradiographically after gel electrophoresis. Functional and metabolic data were acquired after: (1) 8-(4-chlorophenylthio)guanosine-3′,5′-monophosphate (PCPT, cGMP protein kinase agonist) 10 −7 or 10 −5 M; (2) 8-Br-cAMP 10 −5 M followed by PCPT 10 −7 or 10 −5 M; (3) β-phenyl-1, N 2-etheno-8-bromoguanosine-3′,5′-monophosphorothioate, SP-isomer (SP, cGMP protein kinase agonist) 10 −7 or 10 −5 M; (2) 8-Br-cAMP 10 −5 M followed by SP 10 −7 or 10 −5 M. At baseline, percent of shortening (Pcs) and maximal rate of shortening (Rs) were significantly lower in the stunned myocytes (Pcs: 5.0±0.2% control vs. 3.8±0.3 stunned; Rs: 64.8±5.9 μm/s control vs. 46.9±4.8 stunned). In both groups, PCPT and SP dose-dependently decreased Pcs and Rs. The effects were slightly, but not significantly, less in stunned myocytes. 8-Br-cyclic AMP significantly increased function in control, but not stunned myocytes (Pcs, 4.5±0.5 to 6.2±0.8 control vs. 3.1±0.2 to 3.6±0.2 stunned). The negative functional effects of PCPT and SP were diminished after 8-Br-cyclic AMP in control (from −39% to−29%) and diminished significantly more in the stunned myocytes (−19%). PCPT and cyclic AMP phosphorylated similar protein bands. In stunned myocytes, three (22, 31 and 53 kDa) bands were enhanced less by PCPT.