Changes of total and of ribosomal nucleic acids in the endosperm of germinating castor bean seeds. The changes of total RNA and DNA, ribosome RNA and phenol extractable RNA in the endosperm of castor bean seeds during the first 60 hours of germination have been investigated. Total RNA and DNA increase by more than 200% during the germination period investigated. Sucrose gradient fractionation of phenol-extracted and purified RNA showed two distinct peaks corresponding to those commonly found for ribosomal RNA, and a third, minor peak appearently corresponding to soluble RNA. All of these peaks strongly increase with increase of the germination period from 24 to 48 h. Sucrose gradient fractionation of the supernatant of centrifugation at 13,000xg from homogenates from the endosperm shows that the ribosome peak is practically absent at 0 hours of germination, while it rapidly and steadly increases during the following 60 h. These data are put in relation with previous findings on the decrease of total RNA and ribosomal RNA during the latest phases of castor bean seed maturation, as well as with the rapid increase of a number of enzyme systems and structures in the endosperm during the early phase of germination. They are interpreted as showing that a rapid activation of synthesis of the various components of the nucleic acid system plays an important role in determining the metabolic activation of the germinating endosperm.