Escherichia coli heat-labile enterotoxin (Etx) is an oral adjuvant in mice. We show that this is also true for rats. To understand this adjuvant activity we examined lymph dendritic cells (DC) migrating from the intestine to mesenteric lymph nodes (MLN) in animals fed Etx. These DC can prime antigen-specific antibody responses. We show that in rats the small intestine contains 7-24 million DC and 8 x 10(5 )of these migrate to MLN each day. Surprisingly, Etx does not stimulate increased migration of lymph DC. However, oral Etx affects the activation, antigen transport and localization of migratory DC. Specifically, expression of CD25 increases on the CD172a(high) subset of lymph DC. Oral Etx also increases the number of CD172a(high) lymph DC containing co-administered ovalbumin. CD172a(high) lymph DC treated with Etx in vitro, or purified from the lymph of animals fed Etx, stimulate stronger proliferative responses from primed T cells. Etx also directs more of the CD172a(high) lymph DC into the central region of the MLN T cell areas. This change in DC localization is associated with an increase in the expression of CCR7. These data help advance our understanding of the role of DC in initiating mucosal immune responses in vivo.