The activity of ornithine decarboxylase in regenerating rat liver could be completely or partially inhibited in vivo by a single intraperitoneal injection of various amines. Un physiological, 1,3-diaminopropane depressed most effectively the activity of ornithine decarboxylase. It depressed also the activity of adenosylmethionine decarboxylase, which was not inhibited by other amines. The activity of tyrosine aminotransferase was invariably stimulated by injection of the amines. Cycloheximide caused a rapid decay of the activity of liver ornithine decarboxylase (half-life 15 min) and also a decay of the activity of adenosylmethionine decarboxylase (half-life 36 min). 1,3-Diaminopropane inhibited the activity of ornithine decarboxylase (half-life 13 min) and to lesser extent also the activity of adenosylmethionine decarboxylase (half-life 120 min). On the contrary, alpha-amanitin did not have any effect on the activity of the decarboxylases. These experiments are consistent with the view that diamines and spermidine might conceivably control the activity of ornithine decarboxylase in regenerating rat liver in vivo at steps beyond transcription. It is also possible that 1,3-diaminopropane similarly controls the activity of adenosylmethionine decarboxylase thus suggesting that the synthesis of ornithine and adenosylmethionine decarboxylases may be coordinatively regulated in liver.