Nitrogen in cell fractions ofParamecium aureliavaried according to the growth medium. Trichloroacetic acid‐soluble fractions of cells were chromatographer. Adenine, adenosine, guanine, guanosine, hypoxanthine, aspartic acid, glutamic acid, histidine, lysine, proline, and phenylalanine were identified. Fyrimidines and xanthine, or their respective ribosides and ribotides, were not detected. Ammonia was released into the medium by both actively growing and “resting” cells. Culture fluids of “resting”cells also contained hypoxanthine and lesser amounts of adenine and guanine. Urea, uric acid, creatine, cretonne, and ailantoin were absent.Pyrimidine nitrogen seems excreted as dihydrouracil. The following enzymes were detected in homogenates and cell‐free preparations: nucleotidases, nucleoside hydrolases, and cytidine deaminase. Urease, uricase, adenase, guanase, xanthine oxidase, adenosine deaminase, and 5′‐adenylic acid deaminase were not present in this organism.Purine and pyrimidine incorporation into nucleic acids was investigated by the use of radioactive tracers. Guanosine gives rise to nucleic‐acid guanine and adenine; adenosine was precursor to nucleic acid adenine only. Formate was incorporated into purines; glycine was not.P. aureliacan interconvert cytidine and uridine; both give rise to nucleic acid thymine. The methyl group of thymine may be der