Hydrogen peroxide production was measured in non-elicited rat peritoneal macrophages using luminol-dependent chemiluminescence (LDCL). Isolated cells were activated by a chemotactic peptide (FMLP) or by a phorbol ester (PMA) or by the combination of both. A hundred-fold higher LDCL intensity was achieved with PMA relative to FMLP. However, when FMLP was added subsequently to PMA it produced approximately the same response as did PMA. These measurements were carried out with cells isolated from controls and from animals exposed to normobaric hypoxia (10 % O2) for 3 hours, 3 days, or 21 days. Hypoxia had a dual effect. Acutely (within 3 hours) it attenuated the production of hydrogen peroxide triggered by PMA, whilst during longer exposure (3 or 21 days) it increased the response induced by FMLP. Hypoxia can thus modulate the capacity of respiratory burst in peritoneal macrophages.