Alterations in duodenal immunoglobulin (Ig-)producing cells after excessive physical stress (marathon running) were studied by immunohistochemistry in 11 well-trained male adults, both shortly after running (race time, approximately 3.00 h) and later on after a "resting period" of 8-12 weeks with normal training (7-20 h/week). The ratios of IgA-, IgM- and IgG-producing cells were similar in the two biopsy specimens and virtually identical to those in eight normal duodenal controls (medians 77.6% IgA, 18.6% IgM, and 2.5% IgG). However, in the first sample the total number of positive cells per intestinal length unit was increased in five for IgA and in seven for IgM, while it was decreased in eight for IgG compared with the second biopsy. Also, the IgA cell number tended to be slightly increased immediately after the race (median 128 cells/unit) compared with that in normal controls (median 111 cells/unit); the same tendency was found for all Ig classes considered together. This apparent change was not explained by a thickening of the mucosa. Our study showed that marathon runners seem to maintain or even enhance their intestinal IgA and IgM-production, in contrast to the IgA decrease reported for other compartments such as salivary secretions and peripheral blood. The tendency to slightly increased intestinal IgA and IgM immunocyte populations in some runners might reflect a stress-induced hormonal influence on the homing of primed B cells to the mucosa, or perhaps an immune response to elevated influx of stimulatory luminal antigens.