The hematocrit and blood viscosity were measured in ovariectomized (Ovx) rats following thyroidectomy or sham operation. Rats were thyroidectomized (Tx) or sham Tx immediately after ovariectomy. Three months later, Ovx, Ovx+Tx, and intact rats at proestrus stage were bled. Blood was collected by heart puncture in rats under ether anesthesia and heparinized. One month after the first bleeding, Ovx and Ovx+Tx rats were bled once again. the viscosity of whole blood and plasma was measured at various shear rates at 37 degrees C by means of a Mooney-Ewart viscometer. The hematocrit was determined by a microhematocrit centrifuge. The plasma osmolality was measured by a computerized micro-osmometer. Concentrations of plasma sodium and potassium were measured by a flame photometer. The concentration of plasma thyroid stimulating hormone (TSH) was measured by a radioimmunoassay. In Ovx+Tx rats, the plasma TSH concentration was higher, but the hematocrit and blood viscosity were lower than those in Ovx and proestrus rats. The levels of plasma sodium, osmolality and viscosity were not altered by ovariectomy and thyroidectomy. These results suggest that the lower blood viscosity in thyroidectomized rats is due to a reduction of hematocrit.