Cardiac electromechanical intervals were measured during labour in 103 unselected human fetuses. Mean values (+/- SD) for the three intervals measured were 76.01 +/- 9.93 ms for the pre-ejection period, 41.56 +/- 8.51 ms for the isovolumetric contraction time and 156.23 +/- 16.58 ms for the left ventricular ejection time. These are consistent with previously reported data from our own and other centres. the intervals studied did not alter significantly as labour proceeded or during uterine contractions. There was no consistent correlation between them and fetal scalp blood PO2, PCO2 or pH, although the pre-ejection period was shortened considerably in three very acidaemic or hypoxic fetuses, and it was prolonged in association with tight nuchal cord entanglement and variable decelerations in fetal heart rate. These findings raise the possibility of using recorded cardiac electromechanical intervals to determine the clinical significance of different fetal heart-rate patterns that suggest the possibility of fetal distress.