The effects of aging on muscle microvascular structure and function may play a key role in performance deficits and impairment of O2 exchange within skeletal muscle of senescent individuals. To determine the effects of aging on capillary geometry, red blood cell (RBC) hemodynamics, and hematocrit in a muscle of mixed fiber type, spinotrapezius muscles from Fischer 344 x Brown Norway hybrid rats aged 6-8 mo [young (Y); body mass 421 +/- 10 g, n = 6] and 26-28 mo [old (O); 561 +/- 12 g, n = 6] were observed by high-resolution transmission light microscopy under resting conditions. The percentage of RBC-perfused capillaries (Y: 78 +/- 3%; O: 75 +/- 2%) and degree of tortuosity and branching (Y: 13 +/- 2%; O: 13 +/- 2%, additional capillary length) were not different in O vs. Y muscles. Lineal density of RBC-perfused capillaries in O was significantly reduced (Y: 30.7 +/- 1.8, O: 22.8 +/- 3.1 capillaries/mm; P < 0.05). However, RBC-perfused capillaries from O rats (n = 78) exhibited increased RBC velocity (VRBC) (Y: 219 +/- 12, O: 310 +/- 14 microm/s; P < 0.05) and RBC flux (FRBC) (Y: 27 +/- 2, O: 41 +/- 2 RBC/s; P < 0.05) vs. Y rats (n = 66). Thus O2 delivery per unit of muscle was not different between groups (Y: 894 +/- 111, O: 887 +/- 118 RBC. s-1. mm muscle-1). Capillary hematocrit was not different in Y vs. O rats (Y: 26 +/- 1%, O: 28 +/- 1%: P > 0.05). These data indicate that in resting spinotrapezius muscle, aging decreases the lineal density of RBC-perfused capillaries while increasing mean VRBC and FRBC within those capillaries. Whereas muscle conductive O2 delivery and capillary hematocrit were unchanged, elevated VRBC reduces capillary RBC transit time and may impair the diffusive transport of O2 from blood to myocyte particularly under exercise conditions.