Long-term cortisol infusion into freshwater (FW)-adapted eels induced a significant increase in aquaporin-1 (AQP1) mRNA expression within the esophageal epithelium of migratory "silver" eels, but not in nonmigratory, immature "yellow" eels. Cortisol treatment had no significant effect on the mRNA abundance of a second aquaporin-1 isoform, termed AQP1dup, which exhibited a highly variable expression profile among individual members of all fish groups. These results suggest that cortisol, at plasma concentrations similar to that found during FW/seawater (SW) acclimation, induces upregulation in AQP1 expression and thus increases esophageal water permeability during the migration of eels to the SW environment.