Analytical cytophotometry of azure B-stained heart sections was employed to investigate the pattern of myocardial ribonucleic acid (RNA) alterations in rats exposed to acute (1-2 days) and prolonged periods (1-8 weeks) of hypoxia exposure (380 torr). Data support the existence of a slight transient drop in myocardial RNA on day one of exposure followed by a restoration to levels slightly elevated over controls during a 1- to 8-week exposure interval. Because of the high variability in RNA levels among myocytes (coefficient of variation, ca 40%), a shift in the proportion of low and high RNA containing segments of the myocyte population proved to be a more sensitive indicator of suppression or augmentation of RNA synthesis than the use of average RNA levels of the cell population analyzed. Microscopic analyses revealed the presence of compensatory vascular responses which could be effective in ameliorating the extent of tissue hypoxemia, i.e., capillary vasodilation on day 1 with a progressive increase in vascularization with prolonged exposure.