The functional capillary supply of the rat heart was studied in 4-5, 12 and 24 month old animals, respectively. The subepicardial capillaries were visualized by cinemicrophotography of the beating heart in open-chest rats. The intercapillary distances under both normoxemic and hypoxemic conditions were directly measured from focused film frames. The difference between normoxemic and hypoxemic values yielded the degree of capillary recruitment. If one treats all the animals as a single group, then the intercapillary distance decreases under hypoxemic conditions from 17.1 to 16.2 microns which represents an additional recruitment of approximately 11% of capillaries. The degree of recruitment, however, is more pronounced in the youngest age group which is also characterized by shorter intercapillary distances, i.e., higher capillarization than the remaining two age groups (p less than .01). On the other hand, no significant differences between the hearts from one and two year old animals has been found. These results are discussed and compared with those based on morphometric measurements as well as with previous "in vivo" data obtained from the younger age groups.