The effect of transformation by Rous sarcoma virus on the ornithine decarboxylase levels of chick embryo fibroblasts was studied. Infection with Rous sarcoma virus resulted in increased ornithine decarboxylase activity which preceded morphological alterations by at least 24 hr. The ornithine decarboxylase levels of normal controls, unlike those in Rous sarcoma virus-infected cells, declined with age, so that the enzyme activity of transformed cells was increased twentyfold 5 days postinfection. Infection with a temperature-sensitive mutant of Rous sarcoma virus, T5, stimulated ornithine decarboxylase activity of chick embryo fibroblasts at the permissive (37 degrees) but not at the nonpermissive (42 degrees) temperature. The ornithine decarboxylase activity of T5-infected cells shifted from 42 degrees to 37 degrees increased within 2 hr and reached the levels of wild-type, Schmidt-Ruppin-17A-transformed cells in 11 hr. When T5-transformed fibroblasts were shifted to the nonpermissive temperature, ornithine decarboxylase activity decreased fivefold within 4 hr and paralleled the levels of normal controls in 11 hr. Shifting of temperatures of incubation caused comparable alterations in cellular putrescine but not in spermine or spermidine content.