Haemodynamic and heart weight parameters were compared in male rats exposed to intermittent high-altitude (IHA) hypoxia (barochamber, 8 h/day, 5 days/week, total of 24 exposures stepwise up to 7,000 m) starting either from the 4th day or the 12th week of postnatal life. Systemic arterial pressure and heart rate increased in adult IHA acclimatized animals only. Marked chronic pulmonary hypertension and right ventricular enlargement were found in both age groups. Right ventricular weight increased linearly with a rise of pulmonary blood pressure in animals exposed to IHA from the 4th day of life (r = 0.72); no significant relation was found in adult rats (r = 0.16). The close correlation between both variables in young hypoxic rats may be due to the ability of the developing heart to respond to chronic hypoxia by both hypertrophy and hyperplasia of myocytes.