NIH 3T3 cells transformed with the human c-Ha-rasVal-12 oncogene showed markedly enhanced activity of ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis, as compared with their nontransformed counterparts. While in normal and in c-Ha-ras proto-oncogene-transfected cells stimulation with serum caused a transient induction of ODC, in cells transfected with the mutant c-Ha-ras oncogene the activity of ODC persisted at high levels for greatly extended periods of time. The amounts of immunoreactive ODC protein roughly paralleled the changes in the enzyme activity. The augmentation of ODC content by transformation could be largely, but not solely, accounted for by an enhanced accumulation of ODC mRNA. Nuclear run-off transcription assays demonstrated that in transformed cells the rate of transcription of the ODC gene was increased but to a much lower extent than the increase in the level of ODC mRNA. The turnover of ODC mRNA, as measured after actinomycin D treatment, was negligible in transformed cells for up to 8 h, whereas in normal cells the messenger content was initially decreased, by 40% within 4 h, and then remained constant. In normal cells, however, actinomycin D depressed the expression of ODC by more than 80%, while in transformed cells the activity of ODC was slightly superinduced, corresponding to the changes of ODC mRNA. These findings suggest that labile proteins may be involved in the regulation of both the stability and translatability of the ODC mRNA. Transformation led also to about 3-fold stabilization of ODC as determined by an exposure of the cells to cycloheximide. The results thus suggest ODC deregulation at multiple levels in the ras-oncogene-transformed cells.