The synthesis of RNA has been mapped during the initial (dark) phase of germination in cotton seed. During the first 16 hours whole embryos synthesize new ribosomes and protein. Treatment of whole embryos with cycloheximide during this period inhibits protein synthesis and stops further development. Actinomycin D inhibits the synthesis of new ribosomes. This inhibition does not decrease the total ribosomal population, inhibit the synthesis of protein, or retard normal development during this time. These observations suggest that the protein synthesis observed during this period is directed by stable messenger RNA and ribosomes which exist preformed in the mature seed. The synthesis of ribosomes ceases after about 40 hours in the cotyledons and 48 hours in the roots and stems. After this time the synthesis of ribosomal RNA continues but it does not become incorporated into ribosomes. Rather, this RNA is incorporated into a particle, detectable only by radioactivity, which sediments more slowly than monomeric ribosomes on a sucrose density-gradient. On prolonged incubation, the radioactivity associated with the particle moves into mature ribosomes. The particle appears to be an intermediate in the synthesis of ribosomes and has properties similar to those reported for ribosomal precursor particles in bacteria. Its synthesis is completely suppressed by light. During the period of precursor particle synthesis in the roots and stems, protein and RNA synthesis is necessary for further development as determined by inhibition studies with cycloheximide and actinomycin D. The required RNA synthesis is possibly messenger RNA. In cotyledons, during this period, protein synthesis appears to be required for normal development. Whether this protein synthesis is dependent upon concurrent RNA synthesis cannot be evaluated by the use of actinomycin D, probably due to an inability to get the antibiotic into the cells during this period.