1. The effect of a daily injection of testosterone on the in vivo rate of RNA synthesis and degradation in the kidney of castrated mice was determined. 2. Following the administration of testosterone there was a progressive increase in kidney weight, RNA and protein content in the castrated mouse kidney. 3. The rates of synthesis were calculated from the measured incorporation of radioactivity into various RNA species and acid-soluble nucleotides. The kidney RNA was labelled by the injection of a single dose of [5-3H] orotic acid. When the incorporation into ribosomal RNA was at the maximum (48 h), one group of mice was injected with testosterone (100 mug/day/20 g body weight) and the other served as the control. 4. The rates of synthesis and the turnover time were calculated on the basis of the half-life and the rate of decrease of specific radioactivity of RNA. The rates obtained were high in the first 5 days following hormone administration and then slowed down. 5. The results suggest that testosterone changes the rate of synthesis, not the rate of degradation of the mouse kidney RNA. This is consistent with the higher activity of RNA polymerase in testosterone-treated mice as previously observed and described (Avdalović, N. and Kochakian, C.D. (1969) Biochim. Biophys. Acta 183, 382-393; Avdalović, N. (1970) Biochem. J. 119, 331-338).