The technical complexity of previous rheometers has tended to limit the availability of blood viscosity data obtained over a wide range of shear rates. However, an automated tube-type viscometer, the Rheolog, has been developed; it employs a disposable flow assembly and less than five minutes are required to obtain blood viscosity results over a shear rate range of 1-1500 s(-1). We have carried out validation studies of the Rheolog using normal human blood and have compared these results with those obtained by cone-plate and Couette viscometers; storage time and temperature effects were also evaluated. Replicate measurements indicated mean CV levels less than 5%, and were independent of hematocrit and shear rate. Rheolog blood viscosity data agreed closely with those from other viscometers: average Rheolog differences from mean cone-plate and Couette values were -0.3% at 28% hematocrit, -1.4% at 41% hematocrit (i.e., native), and 1.0% at 56% hematocrit. Storage at room temperature up to 8 hours and at 4 degrees C up to 4 days had minimal effects whereas notable changes were observed when stored for 3 hours at 37 degrees C. Our results indicate that, within the hematocrit and shear rate limits employed herein, the Rheolog provides rapid, accurate and reproducible blood viscosity data, and suggest its usefulness for both basic science and clinical studies.