A continuous fetal heart rate tracing of the first and second stage of labour was available for 221 labours and deliveries with varying histories of the present pregnancy. The importance of the abnormalities of the fetal heart rate in these tracings was interpreted in comparison to the development of fetal acidosis. There were no cases of fetal acidosis when changes of the basal frequency occured as an isolated phenomenon independant of the oscillation type. Variable decelerations without silent oscillation of loss of oscillation at the low of the deceleration led to a slight increase of fetal acidosis only after 10 decelerations. Late or combined decelerations were associated with fetal acidosis in 25% of the cases. The incidence of acidosis increase with the increase in the number of decelerations. According to our results the loss of oscillation at the low of the deceleration is of very great importance. Without loss of oscillation at the low of deceleration only 7.2% of the pH values were below normal whereas after 1 to 5 such losses of ascillation already 23.5% of the pH values were below normal. Following more than 10 decelerations with loss of oscillation 75% of the fetuses showed an acidosis. The first abnormal pH values were detectable not earlier than 20 minutes after the first sign of variable, late or combined decelerations in the cardiotocogram. Decelerations in combination with silent oscillation of the basal frequently or loss of fluctuation at the low of the deceleration were associated with fetal acidosis in 50% of the cases. Indications for fetal skalp blood sampling or termination of the labour were proposed in the light of our results.