The specific activity of carbamoyl phosphate synthetase (glutamine-hydrolyzing), the first and rate-limiting enzyme of de novo uridine 5'-triphosphate biosynthesis, was increased in 13 transplantable hepatomas, particularly in the rapidly growing tumors (5.7- to 9.5-fold), and the rise was correlated with tumor growth rates. Thus, synthetase activity was linked with both hepatic neoplastic transformation and progression. Synthetase specific activity was so elevated in a transplantable sarcoma (18-fold) and a kidney adenocarcinoma (5-fold). The increased activity should enhance the capacity of the pathway and should confer selective advantages to cancer cells.