In order to elucidate the mechanisms of plasminogen activator release into the circulation during the first minutes of experimental myocardial ischaemia in vitro, isolated rat hearts were perfused under optimal conditions and under conditions inducing myocardial ischaemia. In ventricular tissues the content of plasminogen a activator was measured; in the perfusate the plasminogen activator and anticoagulating activities were determined. Under optimal conditions the plasminogen activator content in contracting heart was found to be essentially the same as under in vivo conditions; in the perfusates the release of plasminogen activator and of substances possessing the anticoagulating activity was insignificant. Under myocardial ischaemia the release of these substances into the circulation increased, while plasminogen activator content decreased.