Objective: This study was undertaken to determine whether changes in the fetal ST waveform during repeated umbilical occlusion reflect the development of hypotension and acidosis. Study Design: Chronically instrumented, near-term fetal sheep received 1-minute total umbilical cord occlusion either every 5 minutes for 4 hours (1:5 group, n = 8), or every 2.5 minutes until blood pressure fell <20 mm Hg on 2 successive occlusions (1:2.5 group, n = 8). Results: Umbilical cord occlusion caused variable decelerations, with sustained hypertension in the 1:5 group and little change in acid-base status (pH = 7.34 ± 0.07 after 4 hours). In contrast, the 1:2.5 group showed progressive hypotension and metabolic acidemia (pH 6.92 ± 0.1 after the final occlusion). There was a marked increase in ST waveform height during occlusions; this increase was greater in the 1:2.5 group (P <.001), but there was overlap between the groups. ST waveform height between occlusions was significantly higher in the 1:2.5 group (P <.001) until negative and biphasic ST waveforms developed in these fetuses between occlusions in the final 30 minutes. Conclusion: ST waveform elevation occurs during umbilical cord occlusions but only crudely reflects the severity of hypoxia. Interocclusion waveform height may be a better reflection of the severity of hypoxia. The appearance of biphasic and negative waveforms between occlusions may be a useful marker for severe decompensation. (Am J Obstet Gynecol 2001;184:743-51.)