The eosinophil count of 10 rats decreased from a mean resting count of 435.5 cells/cu mm-233.2, 4 hours after swimming to exhaustion (mean swimming time 66.2 min.). After submaximal swimming (mean swimming time 33.1 min.) the mean eosinophil count was 275.3. Following a 6-week period of daily swimming the mean eosinophil count after exhaustive swimming was 114.4 cells/cu mm (mean swimming time 126.5 min.). After submaximal swimming (mean swimming time 63.2 min.) the mean eosinophil count was 143.6 cells/cu mm. Eight days after the termination of training the eosinophil response was measured after swimming bouts of the same duration as the pretraining periods (66.2 and 33.1 min.). The eosinophil counts were 261.2 and 270.0, respectively. Eosinophil counts immediately following training were statistically significantly lower than pretraining counts or counts made 8 days after termination of training. Counts after exhaustive swimming were significantly lower than after submaximal swimming. It was concluded that training increases the magnitude of the eosinophil response to exercise. Submitted on November 13, 1959