We studied the age- and gender-related changes in complexity of arterial blood flow by analyzing the maximum velocity waveforms. Data were collected by insonation of the middle cerebral artery blood flow by a 2-MHz transcranial pulsed Doppler system. Following the paradigm of nonlinear dynamical systems or “chaos” theory, complexity is best evaluated by estimating the correlation dimension ( D 2) and the largest Lyapunov exponent (λ 1). Forty healthy persons (male/female ratio: 1 1 ; mean age 48.6 y; range 19–86 y) were studied. No gender-related differences were observed. The age-effect showed a diminishing λ 1 [ df(1, 36) = 5.687; p < 0.022] and an increasing D 2 at higher age [ df(1, 36) = 4.997; p < 0.032]. The age-related decline of the λ 1 implies a more prominent periodicity, explained by reduced fluctuations in R-R intervals and an altered gain of the baroreceptor reflex. The increased D 2 might be related to more prominent vessel wall oscillations due to the increased vessel wall stiffness at higher age.