Giardiasis has been associated with an increase in allergic disease following infection suggesting an alteration in mucosal immune function. Jejunal in vivo and in vitro macromolecular transport, epithelial permeability, and mucosal and connective tissue mast cell counts were examined in Mongolian gerbils (35-45 g) orogastrically inoculated (I) with a pathogenic strain of Giardia lamblia and compared to age-and weight-matched, sham-treated controls (C) 6 and 21 days postinoculation. Macromolecular uptake was significantly increased in infected tissue at 6 days both in vivo (I 134+/-19 vs. C 74+/-17 ng/hr; n= 8; P< 0.05) and in vitro (I 125+/-17 vs. C 67+/-8 ng/hr/cm; n= 12; P< 0.05). Macromolecular uptake did not differ between groups at 21 days. Infection had no effect on mucosal permeability of [51Cr] EDTA. Mucosal mast cell counts did not differ at 6 days but were significantly elevated in infected tissue at 21 days (I 33.3+/-6.8 vs. C 2.7+/-0.4 per high magnification field; n= 5; P< 0.01) as were connective tissue mast cell counts (I 1.7+/-0.2 vs. C 1.0+/-0.1 per high magnification field; n= 13; P< 0.005). The findings indicate that during the peak phase of giardiasis, jejunal active antigen uptake is increased leading to a delayed recruitment of mucosal and connective tissue mast cells. These changes may play a role in the increased incidence of hypersensitivity reactions associated with Giardia infection.