Long-term experiments employing left ventricular or biventricular bypass with local heparinization and blood filtration for up to 48 days was conducted in 51 calves at different flow rates. The principle of flow control obtained was applied to 15 patients in critical cardiac states. An avcothanized extracorporeal circuit consisted of heparin and protamine microperfusion lines, Pall filter, and nonpulsatile pump. Bypass with flow rates at 30--40% of cardiac output was conducted between 3 and 48 days. Survival was obtained in 34 of 36 calves. Bypass with flow rates at 60--80% of cardiac output was associated with 6 deaths in the 10 calves who underwent the procedure. Bypass with flow rates over 90% of cardiac output was often technically difficult and only 2 of 5 calves survived. Hemodynamic changes correlated well with the above results. In clinical cases flow rate was adjusted up to a maximum of 100% of left ventricular blood flow initially and thereafter to 30--40% of cardiac output for chronic support. A total of 15 patients with myocardial infarction, myocarditis and failing cardiac condition post-surgically were supported for 9--172 hrs. Six of the 15 survived. Adequate flow control in ventricular bypass support is important and the regimen developed in experimental studies proved effective clinically.