Transcription of sea-urchin mesenchyme blastula histone genes after heat shock. Mesenchyme blastula sea-urchin embryos were heat-shocked at 31 degrees C and pulse-labelled with [3H]uridine. The nuclear RNA was fractionated on glyoxal/agarose gels and each RNA fraction hybridized with cloned early blastula histone DNA. The results showed that after heat shock there is an accumulation of histone RNA molecules larger than the messenger and a decrease, with respect to the control, of 9-S histone RNA. Chasing of the heat shock RNA by incubation of the embryos with unlabelled uridine and cytidine restores the radioactivity, as well as the hybridization profiles, of control embryos. Furthermore saturation curves obtained by hybridizing histone DNA with labelled 9-S RNA, showed an absence of the histone messenger in the cytoplasm of heat-shocked embryos, whereas it is present in both control and chased embryos. These results are consistent with the hypothesis that the accumulated histone RNA in heat-shocked embryos is the precursor of mature messengers.