Nonlinear analyses of infant heart rhythms reveal a marked rise in the complexity of the electrocardiogram with maturation. We find that normal mature infants (gestation 2 35 weeks) have complex and distinctly nonlinear heart rhythms (consistent with recent reports for healthy adults) but that such nonlinearity is lacking in preterm infants (gestation ' 27 weeks) where parasympatheticsympathetic interaction and function are presumed to be less well developed. Our study further shows that infants with clinical brain death and those treated with atropine exhibit a similar lack of nonlinear feedback control. These three lines of evidence support the hypothesis championed by Goldberger et al. [Goldberger, A. L., Rigney, D. R. & West, B. J. (1990) Sci. Am. 262,[43][44][45][46][47][48][49] that autonomic nervous system control underlies the nonlinearity and possible chaos of normal heart rhythms. This report demonstrates the acquisition of nonlinear heart rate dynamics and possible chaos in developing human infants and its loss in brain death and with the administration of atropine. It parallels earlier work documenting changes in the variability of heart rhythms in each of these cases and suggests that nonlinearity may provide additional power in characterizing physiological states.Contrary to conventional wisdom, recent evidence suggests that sinus heart rhythms in normal human adults are not strictly regular but have a complex variability that is consistent with nonlinear feedback and possible chaost (1-10). Moreover, such variability appears to be greatly reduced or virtually absent in adults with heart disease and in older adults (>70 yr), where simpler and sometimes cyclic rhythms tend to predominate (7)(8)(9)(10)(13)(14)(15). It is hypothesized that in the healthy adult, chaotic-like dynamics arise from nonlinear feedback created by the interaction of sympathetic and parasympathetic inputs to the sinoatrial node (2-4, 7, 8). According to this hypothesis, heart disease and aging may be associated with a loss of such adaptability and feedback control (8-10). Recent studies of the power spectrum of sinus rhythm heart rate variability by Akselrod and others (16-18) have suggested that this feedback control is not fully developed in infants due to a relative lag in maturation of the parasympathetic influence (19,20). In a similar vein, have shown that heart-rate variability increases in the first three days of life in free-breathing premature infants (27-32 weeks gestation), and Szeto et al. (21) have shown an increase in 1/f complexity of intrauterine "breathing" (diaphragm movements) during maturation of the lamb fetus. Because feedback control should not be present in early gestational infants (22), we reasoned that sequential studies of heart rate variability in the neonatal period might reveal the development of nonlinear feedback and chaotic-like dynamics. Similarly, insofar as balanced autonomic control is needed to produce normal heart-rate variability, we would expect that nonlinearity would be a...