During exercise muscle blood flow increases in proportion to the intensity of activity. In addition to locomotory muscles, aerobic exercise inevitably results in high load on respiratory muscles. This work aimed at testing the hypothesis that the effect of training on muscle blood vessels is also dependent on the functional load on the muscle. Male Wistar rats were treadmill trained 6 days/wk for 8 wk. Efficacy of training was confirmed by 15% increase of max O₂ consumption compared to sedentary control. Isometric contractile and relaxation properties of rings cut from diaphragm (DA) and medial gastrocnemius muscle (GA) feed arteries were studied in vitro. Both arteries had a diameter of 200–250 micron. In controls DA as compared to GA had greater density of sympathetic innervation and higher sensitivity to noradrenaline (NA), but not to serotonin. After training sensitivity to NA did not change in GA, but was reduced in DA; decrease of postsynaptic sensitivity must weaken sympathetic constriction of DA during exercise. In controls dilator responses to acetylcholine (Ach) and sodium nitroprusside (SNP) in DA were also greater than in GA. After training reactivity to Ach was prominently increased only in GA, the effect was abolished after blockade of NO-synthesis (L- NAME, 10^-4M). Interestingly, reactivity of both arteries to SNP was diminished by training indicating smaller sensitivity of smooth muscle to NO. Our results permit the conclusion that differential effects of exercise training on DA and GA are related to basic differences in vasomotor regulation of these arteries. The work was supported by the RFFI (grant 06-04-49699-a).