During acclimatization to sustained hypobaric hypoxia retardation of age-associated decline in left ventricle mechanical activity and improved post-hypoxic recovery was accompanied by upregulation of mitochondrial nitric oxide synthase (mtNOS). To evaluate the time course of regression of these effects upon deacclimatization, rats exposed to 53.8 kPa in a hypopressure chamber for 5 mo were returned to 101.3 kPa, whereas controls remained at 101.3 kPa throughout the study. At three time-points, contractile function in response to calcium and to hypoxia/reoxygenation (H/R) were determined in papillary muscle and NOS activity and expression in mitochondria isolated from left ventricle. Developed tension was, before H/R, 65, 58, and 40%, and, after H/R, 129, 107, and 71%, higher than in controls, at 0.4, 2, and 5 mo of normoxia, respectively. Maximal rates of contraction and relaxation followed a similar pattern. All three parameters showed a linear decline during deacclimatization, with mean half-time (t_1/2) of 5.9 mo for basal mechanical activity and 5.3 mo for post-hypoxic recovery. Left ventricle mtNOS activity was 42, 27, and 20% higher than in controls at 0.4, 2, and 5 mo, respectively (t_1/2 = 5.0 mo). The expression of mtNOS showed similar behavior. The correlation of mtNOS activity with muscle contractility sustained a biphasic modulation, suggesting an optimal mtNOS activity. This experimental model would provide the most persistent effect known at present on preservation of myocardial mechanical activity and improved tolerance to O₂ deprivation. Results support the putative role of mtNOS in the mechanism involved.