Long-lasting cardioprotection may be attained by chronic hypoxia. The basal parameters of contractile function and their response to hypoxia/reoxygenation were measured under isometric conditions in papillary muscles isolated from left ventricle of rats submitted to 53.8 kPa in a hypobaric chamber from 7 wk of age and for their lifetime and of their siblings kept at 101.3 kPa. During acclimatization, hematocrit increased, body weight gain decreased, and heart weight increased with right ventricle hypertrophy. Papillary muscle cross-sectional area was similar in both control and hypoxic groups up to 45 wk of exposure. Developed tension (DT) was 34-64 % higher in rats exposed to hypoxia for 10, 26, and 45 wk than in their age-matched controls, while resting tension was unchanged. Maximal rates of contraction (+T) and relaxation (-T) showed a similar pattern of changes as DT. Recovery of DT, +T, and -T after 60 min hypoxia and 30 min reoxygenation was also improved in adult hypoxic rats to values similar to those of young rats. Heart acclimatization was lost after 74 wk of exposure. Results are consistent with the development of cardioprotection during high altitude acclimatization and provide an experimental model to study the mechanisms involved, which are addressed in the accompanying paper.