Rapid and quantitative polymerase chain reaction (PCR) assays based upon the competitive template technique have been developed for human dihydrofolate reductase (DHFR; E.C.1.5.1.3) and thymidylate synthase (TS; E.C.2.1.1.45) mRNAs. In various tumor cell lines and clinical tumor biopsies, TS mRNA levels correlated with TS levels as determined by [3H]-fluorodeoxyuridylate binding. Levels of DHFR and TS mRNAs, determined by PCR, correlated with mRNA quantitation by conventional dot blot methodology. The ratio of TS/DHFR mRNAs in a number of human carcinoma cell lines varies from 0.4 to 9.9 but ranges from 1 to greater than 1.5 x 10(3) in a number of tumor samples. Differences in the TS/DHFR mRNA ratio in tumors as compared with cultured cells reflects low levels of DHFR mRNA in some tumors. In patients treated with a combination of 5-fluorouracil and leucovorin, mRNA levels for TS increased approximately an order of magnitude in tumor samples 4 and 24 hr after drug treatment, whereas TS levels decreased. These results have significance for the biochemical pharmacology of antifolates and fluorinated pyrimidines in vivo and the relevance of cell culture models for antifolate chemotherapy and drug resistance.