Hemorheologic investigations were performed on nine sheep during thein vivoevaluation of a new axial flow ventricular assist device, the Nimbus AxiPump (Nimbus, Inc., Rancho Cordova, CA). Blood hematocrit, plasma and whole blood viscosity, red blood cell (RBC) deformability and aggregation, plasma fibrinogen, and free hemoglobin (hemolysis) levels were measured. Changes in the main rheologic parameters of sheep blood relative to the pre-implant values were minor and transient. The exception was RBC aggregation, which appeared on the third day of implantation. (Sheep blood does not normally demonstrate RBC aggregation.) Sheep RBCs started to form classic rouleaux typically on the third post-operative day simultaneously with increasing fibrinogen level. To investigate the relative effects of mechanical stress and elevated fibrinogen levels on RBC aggregability,in vitrostudies were conducted with blood from control sheep. These studies indicated that neither mechanical trauma nor elevated fibrinogen alone caused RBC aggregation as seenin vivo.However, combined mechanical stress and elevated fibrinogen did cause this unusual effect for sheep blood.ASAIO Journal1994; 40:959-963.