Capillarity and fiber composition were studied by the ATPase technique in frozen samples of sternothyroid muscle of dogs from sea level (SL) and high altitude (3,300-4,300 m) (HA). Capillary density (CD), capillary to fiber ratio (C:F) and fiber cross sectional area (FCSA) were measured. The mean CD was 791/mm2 at SL and 743/mm2 at HA. CD was linearly related to FCSA in the SL animals (CD=1112.8-0.10 FCSA; r=-0.63). In both SL and HA animals, C:F was linearly and positively correlated with FCSA. There was no significant difference between the two regression lines; therefore, only one line represents all the data (C:F=0.78+(5.19×10-4) FCSA; r=0.77). Thus, at a given FCSA the C:F was the same for SL and HA dogs. Two types of fibers were identified: type I (slow twitch) (42%) and type II (fast twitch) (58%). No differences in fiber composition or FCSA were observed between the SL and HA dogs. These results indicate that moderate levels of hypoxia do not affect the capillarity of dog skeletal muscle. © 1980 Swets & Zeitlinger B.V.