The effect of a single dose of cibenzoline ( (diphenyl 2,2 cyclopropyl)--2 imidazoline, Cipralan ), a new compound with antiarrhythmic properties was studied in 14 patients undergoing routine heart catheterization for suspected coronary artery disease. The effect of the drug on dP/dt, Vmax TP, Vce, negative dP/dt, heart rate (HR), left ventricular systolic pressure (LVSP), left ventricular end-diastolic pressure (LVEDP), cardiac index (CI) and systemic vascular resistance (SVR) was measured before and after drug administration. A significant decrease in left ventricular isometric contraction parameters was manifested immediately after injection, with its maximal effect 2 to 5 min after injection. An increase in HR, a decrease in LVSP, a decrease in CI and an increase in SVR were observed; LVEDP was not significantly altered, nor was negative dP/dt. The effect of the drug on Vmax TP and LVEDP was also examined during two atrial pacing stress tests ( APST ) done before and 10 to 20 min after drug administration. Although the negative inotropic action of the drug was apparent during the second APST , the effect was less pronounced at higher paced heart rates. No difference in the two tests was found between the maximal paced heart rate, nor was there a difference in the angina threshold. Finally the plasma level of the drug and the changes in certain parameters were compared. A positive correlation was found between the plasma level and dP/dt, Vmax TP and cardiac index.