BACKGROUND: The aim of the present study was to investigate the changes in the patterns of power spectrum of R-R interval variability on fetal lamb EKG during acute fetal hypoxia. METHODS: Acutely instrumented six fetal lambs during the third trimester of gestation(115 to 145 days) was subjected to reproducible hypoxia by reduction of maternal placental blood flow with complete obstruction of maternal abdominal aorta for 60 seconds. RESULTS: Power spectral density of low, middle and high frequency increased significantly on hypoxia compared to that of baseline state. Power density of high frequency was significantly higher than that of low and middle frequency in baseline state. And the power density of low and high frequency was significantly higher than that of middle frequency in hypoxic state and 1 minute of recovery. CONCLUSIONS: The conclusion is that an increase in power density of low frequency during hypoxia suggested increased sympathetic activity and increased power density of high frequency throughout the experimental procedures suggested active respiratory movements of fetal lambs close to term and increased parasympathetic activity, reflecting rapidly the autonomic nervous system changes of the fetal lambs.