The effect of parasympathetic (with atropine) and beta-adrenergic (with propranolol) blockade on fetal heart rate variability (HRV) was studied in 162 term fetal Wistar rats. Administration of 1.3 mg of atropine into the jugular vein of the female rat led to an identical decrease of the RR interval in adult rats and in fetuses (about 10%). Administration of 2 mg of propranolol caused a rise in the RR interval by 19% in rats and by 7-9% in fetuses. Overall HRV indices (RMSM, CV) decrease was more pronounced in adult rats than in fetuses. Upon simultaneous administration of both blocking agents the RR interval in adult rats did not differ from the control values; in fetuses it was reduced by 6%. In adult rats HRV indices decreased considerably (by 70%) and their values were within the limits of the estimation error; in fetuses, HRV indices were no higher than upon the administration of any of the blocking agents. Umbilical cord clamping caused a decrease of PO2 in brain tissue from 24.7 +/- 1.7 to 4.7 +/- 0.8 mm Hg within 15 min and led to an immediate increase of the RR interval and the HRV indices if blocking agents were not used. The administration of one or both blocking agents abolished the initial (during the first 7 min of asphyxia) increase of RMSM and CV. Thus, HRV in fetal rats depends to a great extent on the activity of the autonomic nervous system, though it is not fully determined by it. Acute asphyxia causes stimulation of the autonomic nervous system leading to fetal distress.