In nuclei and nucleoli of the slime mold Physarum polycephalum, ornithine decarboxylase (OrnDCase) (Mr 70,000) is phosphorylated by a protein kinase reaction that is dependent on spermidine and spermine. Putrescine antagonizes the phosphorylation. Phosphorylation of OrnDCase inhibits its capacity to catalyze decarboxylation of ornithine. The protein kinase that catalyzes this phosphorylation has many properties similar to those of nuclear protein kinase II, or type G, which has been studied by other groups. The interaction of this protein kinase with OrnDCase resembles the behavior of the OrnDCase antizyme described by other investigators. Phosphorylated OrnDCase binds to purified, palindromic rDNA isolated from nucleoli. It also stimulates transcription of the ribosomal genes by RNA polymerase I in a chromatin form of rDNA. It does not stimulate transcription in a purified, homologous transcription system comprised of RNA polymerase I, rDNA, and phospho-OrnDCase. Thus, phospho-OrnDCase may have a function in promoting rRNA gene transcription but the detailed mechanism is yet unclear. The polyamine-dependent protein kinase and its natural substrate of 70,000 daltons have been demonstrated in other eukaryotic cells, including bovine spermatozoa and rat liver nuclei, and in Ehrlich ascites tumor cells, where the protein kinase is induced by interferon. This phosphorylation system appears to be widely distributed and conserved among eukaryotic species.