During rapid-eye-movement (REM) sleep in adult subjects, the cerebral metabolic rate of oxygen consumption (CMRO(2)) is as high as that during wakefulness. We investigated whether CMRO(2) during active sleep is already at the waking level in newborn life, to support the role of active sleep as a state of endogenous brain activation during early postnatal development. Newborn lambs, 2-5 days old (n = 6), were instrumented with electrodes for sleep-state scoring, catheters for blood sample withdrawal and pressure monitoring, and a transit-time ultrasonic blood-flow probe around the superior sagittal sinus. At the age of 19 +/- 3 days, blood samples were obtained simultaneously from the carotid artery and the superior sagittal sinus during uninterrupted epochs of wakefulness, quiet sleep, and active sleep. The arteriovenous difference in blood oxygen concentration was multiplied by cerebral blood flow to determine CMRO(2). CMRO(2) during active sleep (47 +/- 5 micromol min(-1)) was similar to the value in wakefulness (44 +/- 6 micromol min(-1)) and significantly higher than in quiet sleep (39 +/- 5 micromol min(-1), P < 0.05). These data show that active sleep provides newborn lambs with brain activity at a level similar to that in wakefulness in terms of cerebral oxygen metabolism. The high CMRO(2) during active sleep supports its functional role during early postnatal development, when time spent in active sleep is at a lifetime maximum, albeit constituting a metabolic challenge for newborns, because of the impairment of systemic and cerebral vascular regulation in this sleep state.