The fate and effects of toxicants in aquatic organisms depend not only on magnitude and duration of exposure but also on routes of uptake. To exptrapolate laboratory exposure data to the environment it is necessary to expose animals by environmentally relevant methods. This requires a better understanding of the predominant mechanisms by which exposues occur in the field. In this study the authors demonstrate that tissues specific patterns of hydrocarbon-inducible cytochrome P4501A (CYP1A) expression are dependent on exposure route. Mummichog (Fundulus heteroclitus) were exposed to gradients of enviornmentally relevant concentrations of aqueous and dietary benzo(a)pyrene (BP). Cellular expression of CYP1A was evaluated in longitudinal histologic sections of whole fish by immunohistochemistry. Exposure to aqueous BP resulted in high levels of CYP1A-associated immunohistochemical staining of gill pillar cells, heart endothelium and vascular endothelium.