Four different groups of male Fisher rats were placed on one of the following diets for a period of one month: normal (control) diet, high cholesterol (HC) diet, high saturated fat (HF) diet and low fat (LF) diet. Subsequently, blood samples were drawn and washed in phosphate buffered saline (PBS), and the red cells were resuspended in the same buffer. These samples were introduced into a polycarbonate capillary flow system and their flow characteristics observed. In addition, the mean cell volumes (MCV) in isotonic PBS and the maximum swollen cell volume (MCV
$_{\max}$
) in hypotonic PBS were determined using a Coulter counter. From these, the mean surface area (MSA) and the excess surface area (ESA) were calculated. It was found that rats on a high cholesterol diet develop erythrocyte geometric characteristics which should contribute to improved capillary flow, namely, a decreased mean cell volume and an increased excess surface area. Nevertheless, in these rats a decreased capillary flow was observed as indicated by an increase in mean cell transit time, MCTT, and an appreciable drop in the number of cells per second passing through the capillary flow system. The flow and geometric properties of the high saturated fat and low fat fed rats did not differ significantly from those of the control group.
