In the environment, nonylphenol (NP) occurs predominantly as a degradation product of nonylphenol ethoxylate (NPE). They can be found in many types of products including detergents, plastics, emulsifiers, pesticides, and industrial and consumer cleaning products. As a consequence of their use in a variety of products, they are quite common in rivers and other aquatic environments that receive sewage discharges. Because of its enhanced resistance towards biodegradation, toxicity, estrogenic effects, and ability to bioaccumulate in aquatic organisms NP has been regarded as the most critical metabolite of APEs. We have studied the in vivo and in vitro metabolism and organ distribution of NP in juvenile salmon. Fish were exposed in vivo to waterborne [3H]-4-n-NP for a period up to 72 h or were administered a single oral dose of [3H]-4-n-NP. In vitro biotransformation of NP was studied by exposure of cultured salmon hepatocytes to [3H]-4-n-NP in the presence or absence of a CYP1A-inducer, beta-naphthoflavone (betaNF). Our results show that 4-n-NP was mainly metabolized in vivo, to its corresponding glucuronide conjugates and hydroxylates. The major route of excretion was the bile. The half-life of residues in carcass and muscle was between 24 and 48 h in both waterborne and dietary exposure. In whole body autoradiography, intragastric administered [3H]-4-n-NP was mainly present in the gastrointestinal tract and bile. NP-derived radioactivity in fish exposed via water was more evenly distributed in the organs compared to intragastric exposure and were observed in the intestinal contents, liver, kidney, gills, skin, abdominal fat and brain. In vitro pretreatment of hepatocytes with betaNF had no effect on rates or patterns of NP biotransformation. The in vitro metabolic rate of NP were 118 pmol NP metabolized/h/0.5x10(6) cells without betaNF, and 98 pmol NP metabolized/h/0.5x10(6) cells when betaNF was added to the culture medium.