This study presents the validation of a sensitive method for the determination of 6‐nitrodopa, 6‐nitrodopamine, 6‐nitroadrenaline and 6‐cyanodopamine in Krebs–Henseleit solution by LC–MS/MS with ESI+. HRMS was used to precisely characterize the structures of the fragment ions. The method was applied to investigate the catecholamine basal release from rabbit isolated atria and ventricles. The atria and ventricles were suspended separately in a 5 ml organ bath containing Krebs–Henseleit solution with ascorbic acid (3 mM), gassed (95%O2/5%CO2) at 37°C for 30 min. Strata‐X 33 μm SPE cartridges were employed for the extraction of the catecholamines and the internal standard 6‐nitrodopamine‐d4. The catecholamines were separated employing a 150 × 3 mm Shim‐pack GIST C18‐AQ (3 mm particle size), placed in an oven at 40°C and perfused by 65% of mobile phase A (MeCN/H2O; 90/10, v/v) + 0.4% CH3COOH and 35% mobile phase B (deionized H2O) + 0.2% CH2O2 at 320 μl/min in isocratic mode. The method was linear at 0.1–20 ng/ml. The method was used to identify for the first‐time basal release of the three nitrocatecholamines mentioned above and a member of a novel class of catecholamines, the cyanocatecholamines.