To obtain an estimate of the rate of RNA synthesis by the heterochromatic sex chromatin body, human female fibroblasts were labeled with uridine-5-H(3) and radioautographed. The number of grains over the sex chromatin body was compared with the number of grains over a comparable area of euchromatin. The ratio was 0.37. When corrected for the higher content of DNA per unit area in heterochromatin, the maximum rate of RNA synthesis by the DNA of the sex chromatin body was approximately 18% of the rate of RNA synthesis by a comparable amount of euchromatin DNA. The rate of RNA synthesis by the sex chromatin body did not increase significantly with partial despiralization of this chromatin at prophase.