The RNA synthesized in isolated fat body cell nuclei of blowfly larvae under the influence of ecdysone and juvenile hormone was studied by sucrose density centrifugation and RNA-DNA-hybridization. RNA produced in isolated nuclei of larvae in the early third instar has a polydisperse sedimentation pattern, showing a maximum at about 14 S, while RNA synthesized in nuclei of larvae in a later developmental stage has, in average, lower sedimentation coefficients. No significant differences in sedimentation properties could be seen between RNA of control and hormone-treated nuclei. An investigation of the hybridization of RNA synthesized in hormone-treated nuclei showed that the RNA produced under the influence of ecdysone or juvenile hormone is qualitatively different from that of the controls. This points to a specific effect of the hormones on the genome. The antagonistic effect of ecdysone and juvenile hormone on RNA synthesis is discussed on the basis of hybridization studies.