Adaptation of cardiac muscle to prolonged hypobaric hypoxia (770-740 mbar, 2,250-2,550 m), endurance training, and their combination was studied in rats by investigating the gene expression of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) in atria and ventricles. Rats were assigned into the following groups according to the barometric conditions and physical activity; normobaric sedentary (NS), normobaric training, hypobaric sedentary (HS), and hypobaric training (HT). Experimental periods were 10, 21, and 56 days; the groups at 91 days served as recovery groups from exposure to and training in normobaric and hypobaric conditions for 56 days. The right ventricular hypertrophy in HT rats at 10 days and 56 days was associated with elevated BNP mRNA levels (2.1- and 1.7-fold, P