Effluent discharged from industrial operations often contains a variety of chemicals which can be cytotoxic, mutagenic and carcinogenic in nature. Among these xenobiotic chemicals, the polyaromatic hydrocarbons (PAH) are frequently present in contaminated aquatic environments. Fish absorb these PAH and metabolize them, primarily in liver, by cytochrome P-450 (P-450) - linked mixed function oxygenases (MFO) to polar and reactive products. Increased incidences of hepatic lesions (neoplasms) were observed in fish collected from sites contaminated with high levels of PAH. Laboratory and field studies have shown that PAH and petroleum hydrocarbons are potent inducers of MFO in various fish species. The role of MFO in the biotransformation of PAH and other lipophilic xenobiotics has made the activity of these enzymes a useful biomarker of these pollutants in aquatic species. Because MFO activities in fish tissues can also be altered by various abiotic (e.g. temperature) and biotic (e.g. age, sex) factors, the assessment of accompanying subcellular morphological changes could be useful in delineating induced biochemical effects. In this paper, we have studied these subcellular effects in the liver and kidney of rainbow trout experimentally exposed to 3-methylcholanthrene (3-MC), an MFO inducing agent. Ultrastructural (liver and kidney) and biochemical (liver) indices were usedmore » to demonstrate induced alterations. Such a combined structure-function assessment is rarely used to characterize MFO induction and other systemic effects in aquatic species. 12 refs., 1 fig., 1 tab.« less