In 228 patients, fetal blood pH, pCO2 and lactic acid were measured in two distinguishable parts of the second stage of labor. The ‘first’ part begins at full cervical dilatation and ends when the mother starts her first voluntary bearing down efforts. In our study, the fetal acid-base status did not change in this part, regardless of a late developing hypoxia. In contrast, higher levels of lactic acid and pCOz and lower pH values were observed in the ‘final’ part of the second stage, indicating increasing acidosis. In this ‘final’ part, the fetuses with clinical signs of distress, as defined by an ominous Apgar score at birth, showed quicker and larger acid-base shifts than did the normal fetuses. Thus the two parts of the second stage of labor actually differ in their potential to stimulate fetal acidosis. Since such fetal acidosis may develop especially during the ‘final’ part of labor, we have concluded that special particular attention should be devoted to this part.