A total of 14 healthy subjects [means (SD): 27.6 (3.8) years; body mass 77.8 (6.6) kg; height 183 (6) cm] performed endurance exercise to exhaustion at 100% of the individual anaerobic threshold (Th(an)) on a cycle ergometer (mean workload 207 (55) W; lactate concentrations 3.4 (1.2) mmol.l-1; duration 83.8 (22.2) min, including 5 min at 50% of individual Th(an)). Leucocyte subpopulations were measured by flow cytometry and catecholamines by radioimmunological methods. Blood samples were taken before and several times during exercise. Values were corrected for plasma volume changes and analysed using ANOVA for repeated measures. During the first 10 min of exercise, of all cell subpopulations the natural killer cells (CD3-CD16/CD56+) increased the most (229%). Also CD3+CD16/CD56+ (84%), CD8+CD45RO- (69%) cells, eosinophils (36%) and monocytes (62%) increased rapidly during that time. CD3+, CD3+HLA-DR+, CD4+CD45RO+, CD4+CD45RO-, CD8+CD45RO+ and CD19+ cells either did not increase or increased only slightly during exercise. Adrenaline and noradrenaline increased nearly linearly by 36% and 77% respectively at 10 min exercise. The increase of natural killer cells and heart rates between rest and 10 min of exercise correlated significantly (r = 0.576, P = 0.031). We conclude that natural killer cells, cytotoxic, non-MHC-restricted T-cells, monocytes and eosinophils are mobilized rapidly during the first minutes of endurance exercise. Both catecholamines and increased blood flow are likely to contribute this effect.