The experimental evaluation of the fetal ECG waveform as a method of fetal surveillance has been based largely on experimentally induced hypoxia. The present data, obtained from chronically instrumented fetal lambs, includes three fetuses monitored throughout spontaneous labour, six fetuses with spontaneously developed long-term ST waveform changes and another three fetuses which died in the post-operative period. Uterine contractions could by themselves cause an increase in T wave height (T/QRS ratio greater than 0.30). If the ST elevation was normalised between contractions the fetus seemed to compensate to a moderate degree of hypoxia. When oxygenation was further reduced the T wave remained elevated between contractions and a progressive increase occurred in the T/QRS ratio (greater than 0.60) during the final stages of labour, in parallel with increasing blood lactate levels. Long-term ST changes with T/QRS consistently exceeding 0.30 were found to be related to low haemoglobin levels and/or fetal hypotension. Subsequently, all these fetuses died during labour as compared with a survival rate of 40% in fetuses showing a normal ST waveform. In this group negative T waves are reported as a sign of failing myocardial response to hypoxia. Death in utero, whatever the cause (bleeding, infection or spontaneous hypoxia) was always preceded by marked ST waveform changes. It is concluded that ST elevation with high peaked T waves can appear on a long-term basis in fetuses with intrauterine complications.