The chemiluminescence response of granulocytes to serum opsonized zymosan particles (SOZ) ex vivo was investigated during two ranger training courses lasting 7 days with continuous moderate physical activities corresponding to about 32% of maximal oxygen uptake or 35000 kJ.24 h-1, with energy deficiency (energy supply 0-4000 kJ.24 h-1), and less than 3-h sleep during the 7 days. Significant granulocytosis in combination with a lymphopenia in peripheral blood was observed during the whole course. A priming of the granulocytes for accentuated chemiluminescence response to SOZ was observed during the first days of the course with a maximal increase on day 3 in course A (+35% of control response) and on day 1 in course B (+12%). Thereafter, reduced responses to SOZ compared to control values (-28% and -21% in course A and B) were observed. In course A, a group (n = 8) receiving 5000 kJ.24 h-1 of additional energy, showed a more pronounced priming (maximum +57% versus +21% of control response) during the first days. In course B, all the cadets had 3 h of organised rest/sleep on day 5, and a second priming of the chemiluminescence response was observed on the subsequent 2 days. These data indicated that moderate, continuous, predominantly aerobic physical activities for 1-3 days around the clock primed the production of reactive oxygen species in granulocytes. This priming may be beneficial for, for example, host defence against micro-organisms, but may also contribute to inflammatory damage to normal tissues such as muscle, tendons and joints during exercise. However, when the moderate exercise continued for several more days, a down-modulation of the granulocyte response was observed. The findings of this study further support the possibility that moderate physical activity stimulates immunity, while more extreme duration of the same activities may result in a down-modulation of non-specific (and specific) immunity.