PF9404C is the S-S diesteroisomer of a novel blocker of beta adrenergic receptors with vasodilatory properties. It causes a concentration-dependent relaxation of rat aorta helical strips pre-contracted with 10−6 M noradrenaline (NA; IC50 33 nM). It was equipotent to nitroglycerin (NTG; IC50 49 nM), but much more potent than isosorbide dinitrate (ISD; IC50 15,000 nM).Oxyhaemoglobin (10 μM) shifted to the right the concentration-response curve for the relaxation induced by PF9404C (IC50 530 nM) or NTG (IC50 61 nM).Either methylene blue (MB) or ODQ (1 μM each) largely prevented the vasorelaxing responses to increasing concentrations of PF9404C or NTG.In rat aorta smooth muscle cells, PF9404C increased the formation of cyclic GMP from 3 pmol mg−1 protein in basal conditions, to 53 pmol mg−1 protein in 10 μM PF9404C. Neither metoprolol nor carvedilol enhanced cyclic GMP.In the electrically driven guinea-pig left atrium, PF9404C blocked the inotropic effects of isoprenaline in a concentration-dependent manner. Its IC50 (30 nM) was similar to that for S-propranolol (22.4 nM) and lower than the IC50s for metoprolol (120 nM) and atenolol (192 nM). The beta-adrenergic ligand (−)-[3H]-CGP12177 (0.2 nM) was displaced from its binding to rat brain membranes with Ki of 7 nM, 17 nM, 170 nM and 1.2 μM respectively for PF9404C, S-(−)propranolol, metoprolol, and atenolol.The data are consistent with the idea that the S-S diesteroisomer PF9404C, is a potent vasorelaxing agent, as well as a blocker of cardiac beta adrenergic receptors. The mechanism of its vasorelaxing effects involves the slow generation of NO. This molecule can, therefore, exhibit antihypertensive and cardioprotective actions through a double mechanism, NO donation and beta blockade.