Ornithine decarboxylase (ODC) induction during G1 phase of the cell cycle was compared in Rat-1 fibroblasts and in Rat-1 fibroblasts transformed by the B77 wild-type Rous sarcoma virus (RSV) and by the thermosensitive mutant LA24/RSV. In Rat-1 cells, maximal enzyme activity detectable at mid G1 declined to basal levels by G1-S. The ODC increase was inhibited by actinomycin D and cycloheximide and was dependent on the addition of serum growth factors. Rat-1 (B77 wild-type RSV) cells expressed a greater amount of enzyme activity after serum stimulation, and the maximal level of enzyme activity detectable at mid G1 declined only 50% so that elevated ODC was maintained during G1-S transition. The enzyme increase in the transformed cell line was not dependent on serum growth factors. Fresh medium addition alone induced enzyme activity. Induction in the presence or absence of serum required both RNA and protein synthesis. ODC induction in the transformed cells was less sensitive to repression by exogenous putrescine addition. A 100-fold greater concentration of the diamine was required to produce comparable inhibition in the Rat-1 (B77 wild-type RSV) as compared to the Rat-1 cell. These alterations in the characteristics of ODC expression were found to be a consequence of the transforming function of RSV in the Rat-1 cell line transformed by the thermosensitive viral mutant LA24. In response to serum stimulation at the nonpermissive temperature (39 degrees), Rat-1 (thermosensitive mutant LA24/RSV) cells displayed a discrete G1-phase ODC induction while those cells maintained at the permissive temperature (35 degrees) exhibited a greater and prolonged ODC induction in G1 and S phases. A shift from 39 to 35 degrees in the absence of any medium or serum addition stimulated the induction of ODC expressed in a transformed phenotypic manner. Greater concentrations of exogenous putrescine were required to repress the induction of ODC at 35 than at 39 degrees. Alterations in ODC regulation, therefore, may be inherent to the neoplastic phenotypic change.