Cerebral blood flow (CBF) and autoregulation of cerebral blood flow were assessed in newborn lambs before and after inducing metabolic acidemia. CBF was measured by xenon washout and autoregulation by the change in CBF after increasing mean arterial pressure (MAP) 30 per cent with 0.02 per cent phenylephrine hydrochloride. After surgical preparation, stabilization, and demonstration of the presence of autoregulation, six lambs received 10 mg X kg-1 of lactic acid intravenously over 30 minutes. Arterial pH decreased from 7.35 +/- 0.01 (mean +/- SEM) during the control period to 6.96 +/- 0.02 (p less than 0.01) at the end of the lactic acid infusion. Arterial blood gases, MAP's, and heart rates did not change. Prior to lactic acid infusion, CBF before and after raising MAP were 53 +/- 3 ml/100 g/min and 56 +/- 4 ml/100 g/min respectively. After lactic acid infusion, CBF increased from 54 +/- 2 ml/100 g/min to 62 +/- 3 ml/100 g/min (p less than 0.05) following phenylephrine induced rise in MAP. Five control lambs showed no significant change in any of these variables, including CBF, over the same time periods. These data indicate that metabolic acidosis per se does not alter CBF, but that metabolic acidosis impairs cerebral vascular autoregulation in hyperoxic lambs. Similar changes in human neonates might result in intracranial haemorrhage.