DNA hybridization with nuclear RNA(nRNA) synthesized immediately after ,60,Co Gamma-irradiation in the spleen cells freshly isolated from inbred line 615 mice was investigated, using the technique of Gillespie and Spiegelman (J. Mol. Biol., 12, 829. 1965) .,In RNA/DNA hybridization percentage experiment, it was showed that the hybridization of normal DNA with labelled nRNA synthesized in irradiated cells reached the saturation point at a much faster rate than with labelled normal nRNA (fig. 1) . The hybridization percentage of nRNA synthesized in irradiated cells was higher than that of normal nRNA during the different reaction time before the saturation point of DNA with nRNA synthesized in irradiated cells, but it was lower than that of normal nRNA after the point. It was assumed that the transcription of some base sequences in the zone of high repetitive DNA sequences was stimulated, however, the transcription of some base sequences in the zone of low repetitive DNA sequences was seriously inhibited.,Measurements of the exhaustion rates of pulse-labelled nRNA were carried out as described by Greene and Flickinger Biochim. (Biophys. Acta, 217, 447. 1970) .In these studies, pulse-labelled nRNA synthesized in unirradiated and irradiated cells were compared by exhaustion with DNA at hybridization time of 4 or 24 hours, When the hybridization time was 4 hours, the nRNA synthesized in irradiated cells displayed a faster exhaustion rate than the control nRNA. But if the hybridization time was 24 hours, the exhaustion rate of nRNA synthesized in irradiated cells reduced. These results demostrated that Gamma-irradiation changed the proportion of transcription of some nRNA species and implayed that the sensitivities of the transcription activeties of the different repetitive DNA sequences to Gamma-irradiation were different, and so were the transcription activeties of the different base sequences in the same repetitive DNA sequences. We suggested that Gamma-irradiation stimulated the transcription activeties of some base sequences in the zone of high repetitive DNA sequences which were low before Gamma-irradiation, but Gamma-irradiation inhibited the transcription activeties of some base sequences in the zone of high repetitive DNA sequences which were high before the irradiation.