5''-O-Glucuronides of anticancer nucleosides, 5-fluorouridine and 5-fluorocytidine, were synthesized by 3 different methods. The best preparative procedure was the one starting from benzyl 5-O-(methyl 2'',3'',4''-tri-O-acetyl-.beta.-D-glucopyranosyluronate)-2,3-O-isopropylidene-.beta.-D-ribofuranoside (15) that was obtained almost quantitatively by condensation of benzyl 2,3-O-isopropylidene-.beta.-D-ribofuranoside (8) with methyl (2,3,4-tri-O-acetyl-.alpha.-D-glucopyranosyl bromide)uronate (2). After de-O-isopropylidenation of 15, the crystalline product, benzyl 5-O-(methyl 2'',3'',4''-tri-O-acetyl-.beta.-D-glucopyranosyluronate)-.beta.-D-ribofuranoside (16), was de-O-benzylated catalytically to 5-O-(methyl 2'',3'',4''-tri-O-acetyl-.beta.-D-glucopyranosyluronate)-D-ribofuranose (17). Compound 17 was acetylated to crystalline 5-O-(methyl-.beta.-D-glucopyranosyluronate)-1,2,3-tri-O-acetyl-.beta.-D-ribofuranose (18) and condensed with trimethylsilylated 5-fluorouracil of 5-fluorocytosine in the presence of SnCl4 to afford the corresponding protected nucleosides 5 and 19 in good yields. Saponification of these compounds gave 5''-O-.beta.-D-glucuronides of 5-fluorouridine and 5-fluorocytidine (20 and 21) isolated as their crystalline Na salts. These glucuronides were substrates of both bacterial [Escherichia coli] and bovine .beta.-glucuronidase. They were, as expected, much less toxic against the mouse P318 leukemic cell line in tissue culture.