Ornithine decarboxylase (L-Orbithine carboxylase, E.C. 4.1.1.17) and transglutaminase (R-glutaminylpeptide: amine gamma-glutamyltransferase, E.C. 2.3.2.1.13), enzymes implicated in the regulation of growth processes, were studied in rat placenta during pregnancy; the specific activity of TGase increased from the 15th day until the 21st day of pregnancy while no significant difference of ODC activity was observed; moreover, the ODCase/TGase ratio decreased significantly in rat term placenta.