Blood viscosity (BV) was measured in 32 healthy horses at 6 spindle speeds (60, 30, 12, 6, 3, and 1.5 rpm) and for PCV of 40%, using a digital rotational cone and plate microviscometer. Also, in 7 of 32 horses, BV was measured 3 times each, for 3 PCV values (20, 40, and 60%), and at each spindle speed to determine effect of PCV on BV and machine and among-horse variations. Total plasma protein and fibrinogen concentrations were measured in all horses, using a standard refractometer and heat precipitation, respectively. In 7 of 32 horses, quantitative fibrinogen concentration was measured, using a quantitative fibrinogen assay. Plasma protein and fibrinogen concentrations were measured to determine their effect on BV. Plasma total protein (6.0 to 7.5 g/dl) and fibrinogen (100 to 400 mg/dl) concentrations were within normal reference range for our laboratory.(ABSTRACT TRUNCATED AT 250 WORDS)