This study has examined whether production of superoxide-anion by granulocytes differs between non-pregnant, healthy pregnant and preeclamptic pregnant women. First, we assessed superoxide-anion production in 13 non-pregnant women, 11 healthy pregnant women and 14 preeclamptic pregnant women. Then, we examined the effect of plasma samples of healthy pregnant and preeclamptic pregnant women on superoxide production by neutrophils separated from healthy pregnant women. Superoxide generation was measured by ferricytochrome-c reduction. Phorbol-12,13-dibutyrate- and n-formyl-methionyl-leucyl-phenylalanine-stimulated superoxide-anion production was significantly decreased in healthy pregnant women's granulocytes compared with non-pregnant women. There was no significant difference between granulocyte superoxide-anion production in preeclamptic pregnant and non-pregnant women. When neutrophils from non-pregnant women were incubated in plasma from healthy pregnant women, the granulocyte phorbol-12,13-dibutyrate-stimulated superoxide-anion production was significantly inhibited. With the same stimulator, there were no significant differences between superoxide-anion production of neutrophils incubated in autologous, non-pregnant and preeclamptic pregnant plasma. If n-formyl-methionyl-leucyl-phenylalanine was used for stimulation, there were no significant differences in the superoxide-anion production of granulocytes in either group. Granulocyte superoxide-anion production decreases during pregnancy; this decrease does not occur in preeclampsia, and may cause endothelial damage. It is conceivable that there are unidentified factors in maternal circulation which inhibit superoxide-anion production by granulocytes in healthy pregnant women.