Isolated, perfused guinea pig hearts were used to determine the effects of diacetyl monoxime (DAM) on atrial rate, atrioventricular conduction time (AVCT), and left ventricular peak systolic pressure (LVSP). Contractile force and calcium transients were also monitored in isolated papillary muscles injected with aequorin in the absence and presence of ryanodine (RYA). In the isolated heart, 0.2-5 mM DAM caused significant dose-dependent decreases in LVSP (up to 51%) without change in atrial rate and AVCT. DAM (10 mM) produced a small slowing of atrial rate, no change in AVCT, and a 76% decrease in LVSP; 20 and 30 mM DAM further reduced LVSP, decreased atrial rate 38%, and produced AV dissociation. In isolated, paced papillary muscles, 2, 10, and 30 mM DAM decreased contractile force 27, 58, and 87%, respectively, while calcium transients increased by -9, 38, and 225%, respectively. All cardiac effects of DAM were readily reversible. RYA (1 microM) alone and with DAM (30 mM) decreased contractile force by 30 and 99%, respectively and decreased calcium transients by 59 and 74%, respectively. This study shows that low concentrations of DAM have little effect on electrical activity but greatly depress contractility and modify intracellular handling of calcium.