We evaluated the newly developed miniaturized HIA microdiagonal blood pump (MDP) as a continuous flow left ventricular assist device. In a sheep model (n = 6), the MDP was implanted through left lateral thoracotomy and placed paracorporeally with inflow conduit to left atrium and outflow conduit to descending aorta. The sheep were pumped at a mean flow rate of 2.5 L/min for 7 days. Anticoagulation was applied by intravenous heparin administration. Postoperatively, activated clotting time was held stable with values of 200 seconds. During follow-up, blood samples (creatinine kinase, creatinine, glutamic-oxaloacetic transaminase (aspartate aminotransferase) (GOT), glutamate dehydrogenase (GLDH), gamma-GT, plasma-free hemoglobin, and hemoglobine) were taken daily. After 7 days, the sheep were killed for macroscopic examination. Systemic artery pressures remained stable during the whole test period. Because of operative reasons, the hemoglobin value (7.5 +/- 0.61 g/dl) decreased perioperatively, but recovered within the test period, whereas creatinine kinase increased initially after thoracotomy, but decreased to normal within days. Renal and liver functions were slightly impaired perioperatively, indicated by temporarily enhanced values of GOT, gamma-GT, GLDH, and creatinine. The MDP did not produce significant hemolysis as measured by plasma-free hemoglobin levels. Wound infections did not occur. We conclude that the MDP ran successfully as an left ventricular assist device for 7 days in sheep has potential for long-term support, and may serve as an alternative to current technologies. Presented data were not obtained in a clinical trial; however, the results are promising enough to proceed with longer duration animal studies.