This study was undertaken to determine the possible mechanisms of actions of monensin and digoxin by using isolated guinea-pig ventricular myocytes. Since Ca2+ is the major signal for triggering contraction of cardiac muscle, the objective of this study was to determine whether monensin and digoxin affect the [Ca2+]i of cardiac myocytes and if so is this effect due to an increase in [Na+] i. Three different concentrations of digoxin ( 0.3, 1 and 3 mu mol/l) and three different concentrations of monensin ( 0.3, 1 and 3 mu mol/l) were used. Each treatment was monitored for two hours by using computerized fluoroscopy. Both digoxin and monensin increased the [Ca2+] i and accelerated the onset time of [ Ca 2+] i increase in a dose- dependent manner. Normal myocytes ( loaded with fura-2 for 30 min before the treatment) were also compared with 'weakened' myocytes (loaded with fura-2 for 3 h before the treatment to create a 'weakened' condition). It was found that although 0.3 mu mol/l monensin and digoxin did not change the [ Ca 2+] i in normal myocytes, they increased the [Ca2+]i in 'weakened' myocytes. Finally, a Na+- free medium was used to demonstrate the effect of [Na+](o) on both monensin- and digoxin-induced increases in [ Ca 2+] i. It was found that digoxin did not increase the [ Ca 2+] i in the Na+- free medium. Although monensin increased the [ Ca 2+] i in the Na+- free solution, this increase was not as large as in the Na+- containing medium. The results of the study led to the conclusion that the positive inotropic effect of digoxin depends on [ Na+] o. However, monensin increases [ Ca 2+] i in Na+- dependent and -independent ways. An addition conclusion was that 'weakened' myocytes are more sensitive to the monensin and digoxin treatment than normal myocytes.