1. acetyl-3H- and ethyl-14C-labelled derivatives of phenacetin and related compounds are described. 2. Radioactive label from the ethyl-14C-labelled derivatives of 4-nitrophenetole, 4-phenetidine and phenacetin binds in vitro to various extents to bovine plasma albumin, salmon sperm DNA and yeast RNA; the extent of binding is increased in the presence of a rat liver microsomal hydroxylating system and further increased when the microsomal enymes are induced by prior treatment of rats with 3-methylcholanthrene. 3. The ratios of the bound radioactive labels in vitro from [ethyl-14C]phenacetin, N-acetoxy[ethyl-14C]phenacetin, [acetyl-3H]phenacetin and [diacetyl-3H]N-acetoxyphenacetin per g-atom of DNA P, RNA P and per mol of protein in the absence of the microsomal system are approximately 1:60:11:863, 1:68:41:1835 and 1:88:713:2399 respectively. 4. Radioactive label from labelled phenacetin binds in vitro to all tissues examined, including the spleen, intestines, kidney and bladder; about 80% of the radioactivity bound to the liver is concentrated in the RNA and proteins. 5. Comparison of the relative extents of binding of radioactive label derived from equimolar amounts of labelled phenacetin, ethanol or acetate shows that the incorporation of labelled C2 units into tissues and biological macromolecules in vivo and in vitro may account for only a part of the total bound radioactive label derived from phenacetin and not at all from the incorporation of radioactive acetate into nucleic acids. 6. Some implications of these findings are discussed.

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