['The purpose of this study was to determine the affects of repeated bouts of exercise in the same day on circulating leukocyte number and NK cell activity (NKCA).', 'Ten males (18-25 yr of age) were chosen to complete four trials: two exercise bouts (TB), AM exercise (AM-EX), PM exercise (PM-EX), and control (CN) in a random, counter-balanced order. Exercise consisted of 1-h cycle ergometry, split into three, 20-min segments (5 min at 50% [V0](2peak) and 15 min at 70% [V0](2peak)), and was separated by a 4-h passive recovery. Blood samples were collected from a peripheral arm vein before, immediately, 2 h, and 24 h after the AM bout, and a second series was drawn for the PM bout. Leukocyte subpopulations were measured by manual counting using a hemacytometer and Wright-Giemsa-stained differential slides. NKCA was measured with a whole-blood 51Cr-release assay against K562 target cells. RESULTS Total leukocyte (13.2 +/- 1.2 x 109 L-1, P < 0.035) and neutrophil (7.6 +/- 0.7 x 109 L-1, P < 0.001) counts were significantly higher after two bouts of exercise than a single exercise bout completed in the morning (leukocyte: 7.8 +/- 0.7 x 109 L-1; neutrophil: 5.2 +/- 0.5 x 109 L-1) or afternoon (leukocyte: 9.9 +/- 0.8 x 109 L-1; neutrophil: 5.6 +/- 0.9 x 109 L-1). Lymphocyte counts were significantly greater (P < 0.001) after PM (4.2 +/- 0.5 x 109 L-1) than AM (3.7 +/- 0.4 x 109 L-1) exercise. NKCA was significantly greater (P < 0.001) after exercise in the afternoon (35.3 +/- 8.1%) than morning (27.2 +/- 5.9%).'] Does repeated endurance exercise affect leukocyte number but not NK cell activity?