Doppler velocity waveform analysis from descending aortic and umbilical artery signals was performed in 14 instrumented fetal lambs when umbilical blood flow was altered acutely. Blood flow was increased and reduced (by cord occlusion or placental embolization) and ranged 14% to 168% and 8% to 132% from baseline values in the descending aorta and the umbilical artery, respectively. When flow was severely reduced (less than 50% of baseline value), mean placental resistance increased almost fourfold and mean heart rate fell by 30% from baseline levels. There were only weak correlations between Doppler waveform indices and actual flow, which were usually not significantly different from the correlations between Doppler indices and actual heart rate. With both cord occlusion and placental embolization, significant changes of index values from baseline values were seen only when flow was severely reduced. We conclude that Doppler waveform analysis is not sensitive enough to detect acute mild to moderate reductions in placental flow. Increased index values with severely reduced flow may be explained in part by the concomitant fall in heart rate.