The uptake of 3-methycholanthrene and its metabolism to water-soluble derivatives were both determined in organ cultures of mouse and rat tissues, including prostate, skin, lung and skeletal muscle. All the tissues concentrated the carcinogen from the medium and metabolized part of it to water-soluble compounds. The uptake of tritiated 3-methylcholanthrene was highest in the absence of serum and declined with rising serum concentration. Except for skeletal muscle, it was consistently higher in the murine tissues. The uptake of the hydrocarbon by rat and mouse prostates rose rapidly with time, reaching a maximum after 18 h incubation; the amounts of carcinogen in the tissue then declined and remained at a lower level for the rest of the observation period. The major part of the radioactivity was released within 5 h of transferring the explants to medium without the tracer; 25-40% of the peak concentration of carcinogen, however, still remained in the tissue and further medium changes could not remove any more. Addition of unlabelled 3-methylcholanthrene to the initial incubation increased the radioactivity taken up and caused substantially larger quantities of the carcinogen to be retained after the medium had been changed. The explants converted between 15% and 30% of the 3-methylcholanthrene which they had incorporated to water-soluble derivatives within 48 h but there was no obvious relationship between the amounts of hydrocarbon taken up by the different tissues and the proportions metabolized. A considerable part of the 3-methylcholanthrene in the explants remained unconverted 24 h after its removal from the medium.