Blood count of an athlete is as follows: erythrocytes - 5,5 × 1012/l, Hb-180 g/l, leukocytes - 7 × 109/l, neutrophils -64%, basophils - 0,5%, eosinophils - 0,5%, monocytes - 8%, lymphocytes - 27%. First of all, such results indicate the stimulation of: