The effects of stimulus complexity on the nonlinear electrical brain (EEG) dynamics were investigated in a sample of 24 healthy volunteers. Stimuli used were either a single mechanical low-friction pendulum with a periodic movement (temporal frequency about 1 Hz) or a double-pendulum with a chaotic movement, which were observed for 2-3 min in each case. The prediction that a more complex visual stimulus (double-pendulum) increases the dimensional complexity of brain activity as compared to a simple visual stimulus (single-pendulum), was confirmed by determination of pointwise correlation dimension. Further, there was a significant decrease of alpha power in the double-pendulum compared to a single-pendulum condition. Moreover, a correlation analysis showed a positive correlation between EEG complexity and beta power over the whole cortex in the single- and, above all, in the double-pendulum condition, and also a positive correlation between dimensional complexity and alpha power in the double-pendulum condition only, particularly in the brain regions responsible for the 'bottom-up' sustained attention processes.