A series of substituted 5-aminomethyl-2'-deoxyuridines was synthesized as analogues of 5-thymidylyltetrahydrofolic acid, a proposed intermediate in the thymidylate synthetase catalyzed reaction. 1-(3,5-Di-O-p-toluoyl-2-deoxy-beta-D-ribofuranosyl)-5-chloromethyluracil (3) was treated with the appropriate amine to give the ester protected 5-aminomethyl nucleoside. Removal of the ester groups was accomplished with anhydrous potassium carbonate in methanol to afford the free beta-nucleoside. In this way 5-(2-dimethylaminoethylaminomethyl)-2'-deoxyuridine (5a), 5-dimethylaminomethyl-2'-deoxyuridine (5b), 5-N-mehtylpiperazinylmethyl-2'-deoxyuridine (5c), and 5-pyrrolidinylmethyl-2'-deoxyuridine (5d) were prepared. Compounds 5a,b,d were converted to the respective 5'-phosphates 6a,b,d. All three compounds were subtrate competitive inhibitors of thymidylate synthetase purified from Escherichia coli, calf thymus, and Ehrlich ascites tumor cells. The most active compound was 6a with KI's of 6,3.1, and 14 micronM observed for the respective enzymes.