It is documented that the number of leukocytes in peripheral blood increases considerably during pregnancy. We observed that the range of leukocytosis differs in normal pregnancy differs from that in cases with severe pregnancy induced hypertension (PIH). The data obtained in our university hospital, for example, give the number of leukocytes after twenty-four weeks of gestation as 9,000 +/- 1,600/mm3 (mean +/- SD) for normal cases (93 measurements in 37 cases) and 6,600 +/- 1,100/mm3 for PIH (72 measurements in 18 cases; p less than 0.001). To elucidate the cause and relation to PIH, we studied changes in the activity of leukocytes during pregnancy by means of a modified Nuclepore filtration method (Kikuchi et al., 1983). The time required for leukocytes to squeeze through filter pores under constant suction varies greatly depending on their active states; more active cells show greater resistance to pore passage, which can be clearly shown by exposing cells to various stimuli. The filtration time for 0.5ml blood containing 5,000/mm3 leukocytes minus that of leukocyte poor blood with the same hematocrit was taken as an index of the activity, which we called the rheological activity of leukocytes (RAL). RAL values for blood taken at delivery were 3.4 +/- 1.5 sec for normal cases and 5.0 +/- 2.1 sec for PIH (N.S.). RAL values obtained after the addition of a chemotactic stimulant FMLP (formyl-methionyl-leucyl-phenylalanine) (10nM) to the same samples were 21 +/- 10 sec for normal cases and 405 +/- 265 sec for PIH (p less than 0.001). Leukocyte activity, particularly, response to chemotactic stimulation, was the before thought to be considerably increased in PIH.(ABSTRACT TRUNCATED AT 250 WORDS)