We aimed to determine the time course of postnatal development of ventilatory responsiveness to progressive hypoxia and hypercapnia. Eight lambs underwent hypoxic and hypercapnic rebreathing tests at weekly intervals from soon after birth to 6 weeks of age. Six mature ewes were also studied. During the tests blood samples were collected at intervals from the aorta so that arterial PO2 (PaO2) and PCO2 (PaCO2) could be related to ventilation. Hypoxic sensitivity was defined as the percentage increase in minute ventilation when PaO2 fell from control values (104.7 +/- 6.9 mmHg) to 50 mmHg. When measured in this way, hypoxic sensitivity increased significantly from 64.2 +/- 19.3% (mean +/- SE) in the newborn (2.3 +/- 0.4 days) to 150.4 +/- 14.0% at 3-4 weeks (25.1 +/- 0.6 days, P = 0.05). The hypoxic sensitivity of ewes (66.3 +/- 16.8%) was greatly reduced compared to 6-week-old lambs (140.3 +/- 18.9%, P < 0.05). Hypercapnic sensitivity (ml.min-1.kg-1.mmHg CO2(-1)) did not change significantly with age. We conclude that hypoxic sensitivity increases during the first 3-4 postnatal weeks and declines between infancy and adulthood. In contrast hypercapnic sensitivity does not change with age, although tidal volume and breathing frequency contributions to ventilatory responses change with advancing postnatal age.