Since pregnancy is known to favor systemic generation of reactive oxygen species, this study was designed to assess the levels of exhaled hydrogen peroxide (eH2O2), serum progesterone (PG), 17beta-estradiol (E2) and systemic oxidative parameters in 20 pregnant women between 15th and 28th gestation week and 23 healthy, eumenorrheic women. Exhaled breath condensate H2O2 was assessed fluorometrically with homovanillic acid. Exhaled H2O2 levels were lowered in pregnancy (median Me 0.13 microM) compared with follicular (Me 0.29 microM) or luteal phase (Me 0.26 microM; p<0.05 vs. both). The follicular H2O2 tended to exceed luteal phase. Whole blood chemiluminescence was increased approximately ten fold in pregnancy. E2 markedly decreased chemiluminescence of isolated polymorphonuclear leukocytes. In vitro ferric reducing ability of plasma and 2,2-diphenyl-1-picryl-hydrazyl scavenging assay were not affected by E2 or PG. Decreased exhaled H2O2 during pregnancy, despite of the increased oxidative capacity of peripheral phagocytes, might be ascribed to the magnitude of increased 17beta-estradiol levels.