The Pre-ejection period (PEP) of the cardiac cycle, fetal heart rate, and uterine contractions were monitored in a series of 115 unselected fetus sub partu. PEP was detected by a new on-line technique using EKG and DKG. This method permits a continuous registration of PEP patterns simultaneously with the fetal CTG. Any change in PEP was related to fetal status and fetal heart rate patterns. RESULTS Absolute PEP was prolonged and relative PEP (PEP/cardiac cycle) did not alter during decelerations due to a fetal head compression. The absolute PEP and the relative PEP were prolonged (rarely relative PEP was shortened) during alterations of fetal heart rate patterns based on the alteration in hemodynamics of fetal circulation. A first increase of the PEP was followed by a second increase of the PEP during severe variable decelerations conducting to a secondary chemoreceptor-reflex caused of fetal hypoxemia and acidosis; the duration between the two prolongation peaks of the PEP is directly correlated of fetal blood oxygenation: hypoxemia decreases this duration. These changes are essentially independent of heart rate and therefore PEP can be considered as an independent parameter for fetal assessment.