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Beat-to-beat variability has been suggested as a sensitive indication of fetal well-being. The suggestion originates from case reports of observed decrease in variability preceding clinical signs of fetal asphyxia and fetal death. Against this background a study was performed to evaluate possible changes in heart rate variability in relation to changes in fetal oxygenation. Chloralose-anesthetized sheep were used. Short-term hypoxia was produced without acidosis. The FECGs were recorded on magnetic tape and later analysed by scrutinizing each QRS complex prior to the trigger of a rate meter. The differential index (DI) described by Yeh and co-workers was chosen because it is easily computed and reflects the variability of coefficient of variation for two successive R-R intervals. In contrast to the generally held view of a progressive diminution of variability during the development of asphyxia, the variability was found to increase parallel with a decrease in fetal arterial PO2 determined on blood samples withdrawn at intervals and ranging between 26 and 10 mmHg. The correlation between variability and PO2 in the range below 8-10 mmHg is still to be determined. [ABSTRACT FROM AUTHOR]