Capillarity, O2 diffusion distances and fiber cross-sectional growth were measured in the hearts of guinea pigs exposed early during growth to hypobaric hypoxia (PB = 430 torr, PO2 = 90 torr). Twelve 5-week old males were maintained in a hypobaric chamber for 4-14 weeks. Their hearts were perfusion-fixed via the aorta with a 2.5% glutaraldehyde, 1% formaldehyde buffered solution; blocks were cut from left (LV) and right (RV) ventricles, post-fixed in OsO4, dehydrated and embedded in Spurr medium. Blocks were cut transversely to fiber orientation, 0.5 micron thick, stained with Toluidine Blue, and photographed at 400 X. Number and location of capillaries and fiber cross-sectional areas (FCSA) were scored from these photographs and from those of normoxic controls. Growth rates were similar for control and hypoxic guinea pigs. As animals grew, LV and RV weights increased linearly with body weight. Hypoxic guinea pigs had LV weights similar to controls but the RV showed varying degrees of hypertrophy. Control and hypoxic guinea pigs showed similar linear increases in FCSA with ventricular weight, suggesting that hypertrophy was due to increased FCSA. Capillary density (CD) decreased and capillary-to-fiber ratio (C:F) increased with FCSA, and O2 diffusion distances lengthened in LV and RV of animals in both groups. CD and C:F were higher and O2 diffusion distances were shorter in most hypoxic animals compared to controls. When RV hypertrophy was large (RV greater than 0.7 g) and failure imminent, CD, C:F and O2 diffusion distances were similar to controls suggesting that in these hearts oxygenation was impaired.