Earlier studies showed that administration of MTX to patients leads to an increase in the level of DHFR protein in both normal and leukemic leukocytes as well as in erythrocytes within hours to days.^1,2,3 In vitro studies using a human lymphoblastoid cell line showed that increase in DHFR protein is not transcriptionally mediated but was abolished by cycloheximide treatment, suggesting that new protein synthesis is required.⁴ The rapid increase observed was attributed to either protection of DHFR from degradation by bound MTX and/or dihydrofolate,4 or to an increase in translation of this enzyme.⁵ An increase in thymidylate synthase activity has been reported after 5-fluorouracil treatment,^6–8 and it has been suggested that this increase may also be regulated at the translational level.^7,8 In this communication, we show that DHFR protein regulates its synthesis by exerting an inhibitory influence on its translation and that addition of MTX or dihydrofolate to the reticulocyte translation system relieves this inhibition, thus providing a possible explanation for the rapid increase in this enzyme activity noted in certain cells after MTX administration. In addition, preliminary experiments using CHO cells lacking the DHFR gene and transfected with DHFR cDNA lacking the 5′ leader sequence and treated with MTX showed an increase in DHFR, indicating that this region may not be required for the DHFR protein/RNA interaction.