INTRODUCTION A hallmark of living organisms is that they are not constant in time. Subcellular, cellular, and super cellular processes such as the cycle of cell growth and division, voltage fluctuations in excitable cell membranes, respiration, blood pressure regulation, and the sleep-wake cycle provide spectacular examples of complex rhythms. Interest in these complex fluctuations has been stimulated in recent years by the widespread recognition that deterministic dynamical systems can display chaotic dynamics: aperiodic rhythms sensitive to the initial condition (Abraham et al., 1989; Cvitanovic, 1984; Degn et al., 1986; Glass k Mackey, 1988; Krasner, 1990; Mayer-Kress, 1986). Although analyses of theoretical models and experiments in controlled situations have provided good evidence that chaos can sometimes be found in biological systems (Aihara k Matsumoto, 1986; Chialvo et al., 1990; Guevara, et al. 1981, 1990; Hayashi k Ishizuka, 1986), chaos itself serves more often as a motivating idea for research than an unequivocal scientific finding.