The systemic and local immune responses and the intestinal barrier function were examined in orally or parenterally milk-sensitized guinea pigs. Both types of sensitization led to positive passive cutaneous anaphylactic responses and high IgG titers against beta-lactoglobulin (beta-LG), especially in parenterally immunized animals. In Ussing chambers, sensitized jejunum had higher short-circuit current (Isc) than control jejunum, with and without beta-LG challenge. The further increase in Isc induced by serosal beta-LG treatment was higher in parenterally than orally sensitized animals. Barrier function was tested as the intestinal transport and degradation of horseradish peroxidase (HRP) in the presence and absence of beta-LG. There was a 5-fold increase in degraded HRP transport in sensitized versus control animals, with and without beta-LG challenge. Serosally applied beta-LG enhanced the transport of intact HRP in sensitized but not in control animals. These results indicate that sensitization of guinea pigs to cow's milk permanently increases endocytic and electrogenic activities. The challenge with beta-LG induced a further transient rise in Isc and increased intact HRP transport.