1. A higher concentration of polyamines (spermine, spermidine, putrescine and cadaverine) during development of the chick embryo was observed between the fifth and tenth day of incubation; the concentrations of nucleic acids showed a parallel increase. 2. When spermine (5mumoles) was injected into the yolk sac of embryos at the tenth day of incubation, a high amine-oxidase activity was noted and the spermine and spermidine concentrations were decreased; also, there was a remarkable decrease in RNA and DNA concentrations and a parallel increase in that of total free nucleotides. 3. On the other hand, when iproniazid (16mumoles) was injected there was an inhibition of amine-oxidase activity and a similar increase in the concentrations of spermine and spermidine and of nucleic acids, whereas that of total free nucleotides decreased. 4. Another group of embryos injected with spermine and iproniazid together showed a remarkable increase in spermine and spermidine concentrations and a parallel increase in those of RNA and DNA, and a decrease in that of total free nucleotides.

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1. An increase in polyamine concentration, caused by inhibiting the amine oxidase activities with iproniazid, increased the incorporation of [3H]orotic acid into chick-embryo RNA and DNA. On the other hand, a decrease in polyamine concentration, obtained by causing an increase in amine oxidase activities, decreased [3H]orotic acid incorporation into nucleic acids. This was particularly evident for nuclear DNA and ribosomal RNA. 2. Polyribosomal patterns obtained by sucrosedensity-gradient centrifugation showed highest radioactivity in the regions of 259s and 280s aggregates in those embryos in which the polyamine contents were enhanced, whereas a decrease in the radioactivity was observed when the polyamine concentrations were decreased. 3. The activity of DNA-dependent RNA polymerase, assayed in the same experimental conditions, also varied in the same fashion with changes in polyamine concentration.