A simple rheological test method, as developed by Reid and Dormandy, was applied. In a group of pregnant women (n = 25) without pathological conditions the filtrability of whole blood and of a 10% erythrocyte suspension was determined. The control is a group of healthy nonpregnant patients (n = 7) at different times of the ovulatory cycle. Compared with the control group the flow rate of whole blood in the pregnant state is decreased about 30%, whereas that of the erythrocyte suspension about 40%.