PURPOSE: We investigated the autonomic and complex modulation in heart rate and blood pressure in preterm infants. METHODS: In 20 healthy preterm infants and 20 full-term infants (gestational age of 32.5+/-2.3 and 39.2+/-1.0 weeks each, P<0.0001), we recorded electrocardio gram and non-invasive blood pressure simultaneously and continuously during their sleep in the supine position. To evaluate the autonomic and complex modulation in heart rate and blood pressure, we performed power spectral analysis and analysis of the determinism. RESULTS: When compared to full-term infants and in the analysis of R-R interval, the preterm infants had significantly lower high-frequency power (30.7+/-15.1% vs 20.3+/-7.7%, P<0.05), higher low- to high-frequency power ratio (1.9+/-1.6 vs 3.0+/-2.1, P<0.05), and significantly lower prediction error (4.1+/-2.8% vs 2.9+/-1.3%, P<0.05). In the analysis of blood pressure, they showed no significant difference. CONCLUSION: Compared to heart rates of full-term infants, the heart rates of preterm infants are less modulated by cardiac parasympathetic activity and more deterministic.