In newborn rats after a few minutes of hypoxia, ventilation is similar to the normoxic value. Nevertheless, after a few days in hypoxia, newborn rats have a sustained hyperventilation. In this study we examined the time course of the newborn rat's adaptation to hypoxia. Measurements of body size, hematocrit, lung and heart mass, and breathing pattern have been performed on newborn rats exposed to hypoxia (10% O2) for different time intervals from 4 to 60 h (hypoxic, H), and on same-age rats growing in air (controls, C). Ventilation measured by flow plethysmography was increased in H rats above the C value from about 8 h; this was due to a higher breathing rate and, from 24 h, also to a larger tidal volume. During the early hours of hypoxia, oxygen consumption measured manometrically was about 50% of C, while after 3 days in hypoxia it was almost like the C value. These observations indicate that the lack of sustained hyperventilation, characteristic of the newborn's acute exposure to hypoxia, is an immediate but transient phenomenon that is resolved after a few hours, and suggest a tight link between metabolic and ventilatory hypoxic responses. Body weight of H rats was less than in C, owing to an immediate decrease below the prehypoxic value. Dry heart and lung weight changed in proportion with the rest of the body during the first 36-48 h of hypoxia, then they increased disproportionately more. Hence, these temporal changes suggest that the large heart and lung weight-body weight ratios of the chronic hypoxic animals result from their smaller body mass and the stimulated growth of cardiac and pulmonary tissues.