Heart rate variation (HRV) was measured in 12 children with brain death. Computer analysis demonstrated an overall heart rate variation (RMSM) of 3.5 to 9.6 msec (mean, 6.0 msec). The respective figures for beat-to-beat variation (RMSSD) were 4 to 16 msec (mean, 7.7 msec). Normal infants (RMSM, 20 to 30 msec) and children (RMSM, 50 to 90 msec) and infants with respiratory distress syndrome (RMSM, 10 to 20 msec) have a clearly higher HRV than those with brain death. In all subjects a periodic component of the HRV with a cycle length of 5.9 +/- 0.26 seconds was discovered. In five patients an additional periodic component was found that corresponded exactly to the rate of artificial respiration. This study demonstrated the value of accurate digital techniques in the characterization of fixed heart rate in decerebration. This phenomenon could be considered one clinical criterion of brain death.