We have miniaturized and optimized our implantable rotary blood pump developed to provide long-term mechanical right heart support for patients who have failing Fontan circulation. The objective of this study was to evaluate the miniaturized Fontan circulation assist device (mini-FCAD) during 30-day sheep studies (n = 5). A complete right heart bypass was performed and all return flow was supported by the pump. Postoperatively, unfractionated heparin was given to maintain thromboelastography R times of 2× normal. The first two studies were terminated on day 0 and day 4 due to complications. In the final three studies, the animals remained healthy and were electively terminated at 30 ± 2 days. Pump flow was between 5 and 7 lpm, left atrial pressure remained normal, and inlet pressures were between 3 and 18 mm Hg with no incidents of suction. There was no evidence of hemolysis, end organ or pulmonary dysfunction, thromboembolic events, nor thermal damage to the surrounding tissue. Explanted devices from two studies were free of thrombi and in the third study there were unattached thrombi on the SVC inlet of the rotor. The mini-FCAD was successfully tested in vivo as a right heart replacement device demonstrating adequate circulatory support and normal physiologic pulmonary and venous pressures.