A series of 5,8-dideaza analogues of folic acid, isofolic acid, aminopterin, and isoaminopterin were evaluated for inhibition of thymidylate synthase, TS, from mouse L1210 leukemia cells with 10-propargyl-5,8-dideazafolic acid, CB3717, 4a, as the reference inhibitor. These compounds were also tested as inhibitors of human dihydrofolate reductase, DHFR, obtained from WIL2 cells. None of the analogues studied were as potent as 4a toward TS; however, 9-methyl-5,8-dideazaisoaminopterin, 6d, was only 2.5-fold less effective. Compound 4a was prepared by direct alkylation of the di-tert-butyl ester of 5,8-dideazafolic acid followed by hydrolysis of the resulting diethyl ester, which resulted from concomitant transesterification. It was found to be identical with a sample of 4a prepared by earlier methodology by using a variety of spectroscopic techniques. Its isomer, 9-propargyl-5,8-dideazaisofolic acid, 4b, which was synthesized by an analogous approach, was found to be dramatically less inhibitory toward TS than 4a. Each of the 2,4-diamino derivatives, including those possessing an allyl or propargyl group at N9, was an excellent inhibitor of DHFR, having a level of potency similar to that of methotrexate, MTX. However, many of these 5,8-dideazaaminopterin analogues were far more inhibitory toward TS than MTX.