Mummichog (Fundulus heteroclitus), an estuarine, teleost fish, were exposed for 456 hr to environmentally relevant concentrations of aqueous (10 micrograms/liter) and dietary (10 micrograms/g) benzo[a]pyrene (BP) in static renewal aquaria. Cellular expression of BP-inducible cytochrome P4501A (CYP1A) was evaluated several times during exposure by immunohistochemistry in longitudinal histologic sections of whole fish. CYP1A-associated staining intensities in tissues were scored by a subjective rating system similar to that used previously for qualitative information. Exposure to aqueous BP resulted in high levels of CYP1A-associated immunohistochemical staining in gill pillar cells, heart endothelium, and vascular endothelium. Exposure to dietary BP resulted in only mild to moderate staining in these tissues but high-intensity staining in gut mucosal epithelium. CYP1A induction in hepatocytes appeared most sensitive to aqueous exposure. Route-specific patterns of CYP1A expression were also observed in other cells including gill epithelia, pseudobranch, and skin. Expression of CYP1A in renal tubules and interrenal tissues was not affected by either treatment. Coexposure to both aqueous and dietary BP resulted in a pattern of induction reflecting both routes of exposure. In addition to the subjective rating system for scoring CYP1A expression, we developed a photometric approach that was used to obtain quantitative data on CYP1A-associated staining intensity. Photometric values of CYP1A staining intensity revealed patterns essentially the same as those observed during subjective ranking but were amenable to statistical analysis. The results of this study support the use of tissue-specific patterns of CYP1A expression in identification of target sites and exposure routes for polycyclic aromatic hydrocarbons and other compounds.