The purpose of this study was to examine the impact of intensive training for competitive sports on natural killer (NK) cell lytic activity and subset distribution. Eight female college-level volleyball players undertook one-month of heavy pre-season training. Volleyball drills were performed 5 hours per day, 6 days per week. Morning resting blood samples were collected prior to training (PRE), on the 10th day of training (DURING), one day before the end of training (END) and one week after intensive training had ceased (POST). CD3^-CD16^brightCD56^dim (CD56^dim NK), CD3^-CD16^dim/-CD56^bright NK (CD56^bright NK) cells and CD3⁺CD16^-CD56^bright (CD56^dim T) cells in peripheral blood were determined by flow cytometry. The circulating count of CD56^dim NK cells (the predominant population, with a high cytotoxicity) did not change, nor did the counts for other leukocyte subsets. However, counts for CD56^bright NK and CD56^dim T cells (subsets with a lower cytotoxicity) increased significantly (p<0.01) in response to the heavy training. Overall NK cell cytotoxicity decreased from PRE to END (p=0.002), with a return to initial values at POST. Lytic units per NK cell followed a similar pattern (p=0.008). Circulating levels of interleukin-6, interferon- and tumor necrosis factor- remained unchanged. These results suggest that heavy training can decrease total NK cell cytotoxicity as well as lytic units per NK cell. Such effects may reflect in part an increase in the proportion of circulating NK cells with a low cytotoxicity.