The different stages of the phagocytic function in peritoneal macrophages from old guinea pigs (27 +/- 3-months-old) were studied before, immediately after and 24 h after being subjected to physical activity stress (swimming until exhaustion) which raised the blood levels of corticosterone. The phagocytosis of opsonized Candida albicans was stimulated immediately after physical activity. No modifications in adherence, chemotaxis, ingestion of inert particles, or microbicide capacity, measured by nitroblue tetrazolium (NBT) reduction, were found. At 24 h, when no stress could be shown by corticosterone analysis, the phagocytosis of opsonized C. albicans remained stimulated and chemotaxis was increased while ingestion of inert particles and microbicide capacity remained unchanged. The adherence, however, was at a smaller level. No correlations were found between the corticosterone levels and the status of the phagocytic process of peritoneal macrophages.