Accumulation of the putative drug FG 9202 in isolated rabbit hearts showed monophasic exponential kinetics with a half-life of only 0.59 min. The disposition showed a three-phasic exponential time course with half-lives of 0.34, 1.51 and 15.8 min, respectively, which was interpreted as three-compartment kinetics. FG 9202 accumulated only about 3 times in the myocardium at steady-state with 51, 32 and 17% referable to a superficial and two deeper myocardial drug pools. The drug did not significantly affect contraction amplitude or velocity of contraction at increasing concentrations up to 40.6 micrograms.ml-1 (119 microM). Heart beating frequency decreased slightly but only significantly at some of the higher concentrations. Neither dromotropic, bathmotropic nor ischaemic ECG-effects were observed. Coronary flow-rate and myocardial oxygen consumption decreased at the highest drug concentrations. Myocardial efficiency expressed as the ratio of contractile parameters to oxygen consumption showed a minor but insignificant increase at the highest drug-exposure levels. Our findings indicate that FG 9202 is not potentially toxic to the isolated, spontaneously beating rabbit heart in vitro.