The present study evaluates the strength of the drive to breathe and the respiratory response of newborns to metabolic acidosis. The study consisted in a HCl mild and severe acidosis induced in 30 unanesthetized newborn lambs aged from 2 to 90 days. During mild acidosis (pHa = 7.20), all lambs experienced an increase in the minute ventilation (VE) associated with an increase in the neuromuscular drive of breathing which was measured by the occlusion pressure (PO.1). However, with severe acidosis (pHA = 7.10) both VE and PO.1 decreased towards the preacidotic level in the 2- and 10-day-old lambs but continued to increase in older lambs. It is concluded that newborn lambs have an immature drive of breathing and a blunted respiratory output to metabolic acidosis.