The subepicardial capillaries were visualised under normoxemia and hypoxemia by cinemicrophotography of the beating heart in open-chest rats. The functional intercapillary distances (ICD) were directly measured from focused frames. Under normoxemia, the average ICD was 19.2 micrometers and under hypoxemia, 17.9 micrometers (p less than 0.01). This decrease of 1.3 micrometers under hypoxemia corresponds to an average recruitment of an additional 416 capillaries/mm2 (from 2762 to 3178). During postnatal development, both the normoxemic and hypoxemic ICD's increased with left ventricular weight. The regression analysis of this relationship indicates that the hypoxemic ICD's remain significantly shorter within the observed range. In heterotopically isotransplanted, empty, beating rat hearts, there was no difference in functional ICD between normoxemia and hypoxemia either in 1-day-old (18.5 versus 18.6 micrometers) or in 7-day-old (17.1 versus 17.1 micrometers) transplants. Unlike the hearts in situ which could be observed only through a long working distance, low power objective (11 X), the heterotopic transplants could also be observed through a higher power (22 X) lens with an optical penetration of less than 5 micrometers. On the same hearts almost identical ICD's were obtained by both lenses during normoxemia and hypoxemia. Therefore, recruitment of capillaries observed on the normal hearts could not be due to different optical characteristics of the hypoxemic myocardium.