Nuclei were isolated from the anterior pituitary glands of male rats with the aid of Triton X–100. An average recovery of 90% was achieved. In the presence of all four nucleoside triphosphates and magnesium ions, these nuclei incorporated [3H]nucleotides into an RNA product with a base composition similar to ribosomal RNA. Incorporation was inhibited by pyrophosphate, actinomycin D and deoxyribonuclease. These properties are characteristic of RNA polymerase activity. Several aspects of the assay procedure were studied, including incubation time, divalent cation concentration, pH and nucleoside triphosphate concentration, and the optimal conditions for assay of pituitary polymerase were determined. Pituitary nuclei isolated from adult males were about three times as active as liver nuclei. The level of enzyme activity in pituitary nuclei varied somewhat with age. Activity was 60% greater in 56–day—old rats than in 2 5–dayold rats, but no further increase was found in, 100–day—old rats. (Endocrinology92: 1790, 1973)