Cerebral blood flow velocity was studied during changes (haemorrhage) in mean arterial blood pressure or P(a)CO2 in 56 (aged 0-26 days) anaesthetized and ventilated piglets. The CO2 reactivity increased with age from 6.5% kPa-1 (< 1 day) to adult levels of 25% kPa-1 for piglets over 4 days old. The mean arterial blood pressure reactivity was reduced from 1.3% mmHg-1 (< 1 day old) to 0.0%/mmHg (> 4 days old). The reactivities were similar with two different anesthetics: chloralose/urethane or pentobarbital. To validate the cerebral blood flow velocity data, both electromagnetic flow and precerebral Doppler ultrasound velocity were recorded from the same common carotid artery with extracranial branches tied off. There were no differences between the results with these two methods nor between these results and those obtained when the cerebral blood flow velocities were recorded from an intracerebral artery and the electromagnetic flowmeter recorded from the carotid artery. The vessel diameter appears stable during these interventions. In conclusion, the autoregulatory response and the reaction to P(a)CO2 appear poorly developed in the newborn piglet, but rapidly mature during the first 4 days of life.