The study was undertaken to ascertain whether variations in maternal ventilation, designed to produce respiratory acidosis and alkalosis in the mother, could influence umbilical venous carbon dioxide content and whether there was a correlation between carbon dioxide values and onset of effective spontaneous respiration in the infant. Neonatal acidosis was studied both with and without central depression in order to ascertain whether carbon dioxide was capable of stimulating the respiratory centre under either or both conditions. The results suggest that respiratory acidosis stimulates respiration in the newborn and the alkalosis has the opposite effect.