In this study we investigated centrally mediated parasympathetic regulation of modulated cardiac vagal tone among children with severe cyanotic and pallid breath-holding spells by examining respiratory sinus arrhythmia. Respiratory sinus arrhythmia was evaluated in 41 children; 17 subjects with cyanotic breath-holding spells (6 boys, 11 girls; mean age 37.1 months), 7 subjects with pallid breath-holding spells (2 boys, 5 girls; mean age 33.0 months), and 17 controls (8 boys, 9 girls; mean age 41.2 months). Subjects had recurrent (more than 3) severe breath-holding spells. Each subject's electrocardiogram was recorded in a quiet room and digitized by an 80386 personal computer during five 1-minute periods. R-R intervals within each 1-minute period were converted to heart rate in 120 successive 0.5-second intervals. The resultant heart rate time series was converted to its underlying frequency composition by a fast Fourier transform and averaged across minutes. Respiratory sinus arrhythmia was defined as the variability in the time series over a frequency range (0.096 Hz to 0.48 Hz) corresponding to a range of respiratory rates from 6 to 30 breaths per minute. ANCOVA adjusting for age and sex was conducted with the subject group as the independent measure. There were no significant differences between subjects with cyanotic breath-holding spells and controls. Pallid breath-holding spell subjects had a marked difference in respiratory sinus arrhythmia from either controls or subjects with cyanotic breath-holding spells, demonstrating less variability in respiratory sinus arrhythmia (P < .042) This study supports the hypothesis that there exists autonomic dysregulation in pallid breath-holding spells, caused by a primary central parasympathetic disturbance distinct from the dysregulation found in cyanotic breath-holding spells.