Effects of longitudinal exercise training and acute intense exercise (simulated race test) on immune function have not been reported in horses. This study investigated the interaction of 8 week of exercise training with or without 12 week of clenbuterol administration in horses. Twenty-three untrained standardbred mares were used and divided into four experimental groups. Horses given clenbuterol plus exercise and clenbuterol alone received 2.4 ìg/kg BW of clenbuterol twice daily on a schedule of 5 days on and 2 days off for 12 weeks. Mares given exercise alone were aerobically trained for 3 days/week, and the control group remained sedentary. Lymphocyte proliferative response was not affected by clenbuterol or exercise treatment. From the present experiment it was apparent that horses demonstrated responses to bouts of acute exercise as noted with other species, namely humans and rodents.