Fifty-five children had 64 external ventricular drains (EVDs) placed predominantly (95%) for cerebrospinal fluid (CSF) shunt infections. In 9 children, a computer monitoring system measured the CSF output each second continuously for up to 24 hours. The monitoring was repeated daily for up to 9 days. The state of arousal of the patients was recorded simultaneously. In all children, daily EVD outputs were related to age, sex, weight, method of establishing the EVD, height of the drip chamber, time since insertion, and type of infecting organism. Computer monitoring revealed wide fluctuations in flow rate, with peak rates frequently greater than 20 ml/h and periods of flow arrest. These changes were usually associated with increased arousal, but also occurred with sleep. The mean EVD flow rate for all children was 6.3 ml/h. EVD output increased with age and weight. EVD output decreased with Gram-negative or multiple-organism infections and with elevation of the drip chamber. Resolution of the infection, sex of the patient, and method of establishing the EVD had no effect on output. These results predict that CSF production increases with brain growth in humans: that CSF production is depressed by Gram-negative and multiple-organism infections: that implanted CSF shunts with standard valves flow at equivalent rates to an EVD in the supine position; and that the CSF drainage requirements in this group are approximately equal to their EVD outputs.