The effect of muscle contraction, and extension, on capillary anisotropy was investigated in rat m. soleus fixed by vascular perfusion at sarcomere lengths ranging from 1.62 micron (tetanizing stimulation of sciatic nerve) to 2.85 micron (ankle joint maximally flexed. Capillary length density and tortuosity were estimated by morphometry using two sets of sections (0 and 90 degrees to the fiber axis). Capillary orientation distribution was evaluated from a series of sections taken at 0 to 90 degrees (by steps of 5-10 degrees) to the fiber axis in six preparations (sarcomere length range, 1.62-2.85 micron; capillary length density, 900-2000 mm-2). The Fisher axial distribution provided a good fit for modeling capillary orientation distribution in each case. For a comparable capillary length density per volume of muscle fiber (approximately equal to 2000 mm-2), the degree of orientation of capillary segments parallel to the fiber axis was two and four times larger in extended m. soleus than in the muscles fixed at 1.98- and 1.62-micron sarcomere lengths, respectively. In preparations fixed at 2.85, 1.98, and 1.62 micron, capillary length density per volume of muscle fiber was, respectively, 14, 44, and 65% larger than revealed by capillary counts per sectional area of muscle fiber on transverse section only, an often used parameter to compare capillarity in different muscles.