Respiratory gases and acid base parameters were determined in the samples of fetal blood collected by cordocentesis from 70 patients with normal course of pregnancy, between 18th and 38th gestational week. The mean pH value was 7.386 +/- 0.05; partial pCO2 pressure -4.819 +/- 1.464 kPa; O2-5.712 +/- 1.24 kPa; bicarbonate 21.528 +/- 0.494 mMol/l; base excess -2.284 +/- 0.647 mMol/l and saturation 74.86 +/- 13%. Statistically significant negative correlation between gestational age and pH (r = 0.841, p less than 0.01) and partial O2 pressure (r = -0.961, p less than 0.01) was revealed. Partial CO2 pressure correlated significantly with gestational age (r = 0.923, p less than 0.01). The values of bicarbonate and base excess do not change significantly during gestation. Our own standards of examined parameters were formed.