Heterogeneity of capillary spacing may affect tissue oxygenation. The determinants of heterogeneity of capillary spacing are, however, unknown. To investigate whether 1) impaired angiogenesis and increased heterogeneity of capillary spacing delays development of hypertrophy during aging and 2) heterogeneity of capillary spacing is determined by variations in fiber size we overloaded the left m. plantaris in young-adult (5-month-old) and old (25-month-old) rats for 1, 2 or 4 weeks by denervation of synergists, while the right leg served as an internal control. Fiber size, capillary density and capillary to fiber ratio were similar in control young-adult and old muscles. The time course and degree of hypertrophy were similar at both ages, indicating that in rats up to the age of 25 months the hypertrophic response is maintained. The variation in fiber size and heterogeneity of capillary spacing were, however, larger in old than young-adult muscles, larger in the superficial than the deep region of the muscle, and correlated significantly (R = 0.558; P < 0.001). This suggests that part of the heterogeneity of capillary spacing is due to heterogeneity in fiber size and may reflect that morphological constraints for positioning of capillaries partly determines heterogeneity of capillary spacing in muscle.