Tissue hematocrit was determined in left ventricular myocardium slices in in situ frozen rat hearts and the influence of experimental plasma fibrinogen increments was investigated. Hematocrit values in 100 mu m thick myocardial slices were estimated by measuring the activity of two different radionuclides labelling plasma (I-125-Albumin) and red blood cells (Tc-99m). In the control group with physiological levels of fibrinogen, myocardial tissue hematocrit was highest at the layer adjacent to epicardium. It decreased linearly through the depth of myocardium, approaching a minimum at the myocardial layer closest to the endocardium. In the group with about two folds higher plasma fibrinogen concentration, plasma viscosity and red blood cell aggregation were about 50% higher. In this group, tissue hematocrit difference between epicardial and endocardial layers existed, however this difference was more likely due to a step change, rather than a linear gradient. These findings confirm those physiological mechanisms underlying the hematocrit reduction in small blood vessels might be affected by hemorheological alterations.