Methylflavonolamine hydrochloride (4'-methyl-7-(2-hydroxy-3-isopropylamino-propoxy)-flavone hydrochloride, MFA) is a new compound synthesized by Shanghai Institute of Pharmaceutical Industry in China. Its effects on the fast and evoked slow response action potentials of isolated guinea pig papillary muscles have been studied by intracellular microelectrode techniques. The data were automatically analysed by a microcomputer system. At concentrations of 5-75 mumol/L, MFA caused a significant decrease in the duration required for 30, 50, 90% repolarization and a shift in the plateau to more negative transmembrane potentials. MFA 5 mumol/L had no effect on the maximal upstroke velocity during phase 0 (Vmax) but decreased it at 75 mumol/L. MFA had no significant effects on other AP parameters. It significantly inhibited the slow AP induced by isoprenaline or histamine (1 mumol/L) after myocardial cells were depolarized in high K+ (25 mmol/L) solution. The results suggest that MFA inhibits the inward Ca2+ current and at high concentration inhibits also the Na+ current.