A novel fluorescent labelling agent, 5-dimethylamino-2-oxidoisoquinolin-1-yl diazomethane (3) was designed and synthesized for the fluorescent labelling of the phosphate moiety of nucleotides and nucleic acids. Starting from 1-cyano-5-nitroisoquinoline (4), 1-carboxy-5-nitroisoquinoline (5) was obtained after hydrolysis with hydrochloric acid. Esterification of 5 with methanol in the presence of sulfuric acid afforded 1-methoxycarbonyl-5-nitroisoquinoline (6). Catalytic hydrogenation of 6 followed by treatment with formic acid-acetic anhydride gave the 5-formamido derivative (8). Methylation of 8 with methyl iodide in the presence of sodium hydride afforded the 5-N-methylformamido derivative (9). Reduction of both the ester group and formyl group with aluminum hydride followed by treatment with chloranil and acetic anhydride provided 1-acetoxymethyl-5-dimethylaminoisoquinoline (11). N-Oxidation of 11 with m-chloroperbenzoic acid followed by selective removal of the oxido group at the 5-position by reaction with carbon disulfide afforded 1-acetoxymethyl-5-dimethylaminoisoquinoline-2-oxide (13). After deacylation of 13, selenium dioxide oxidation of the hydroxymethyl group followed by reaction with p-toluenesulfonyl hydrazide gave 5-dimethylamino-1-formylisoquinoline-2-oxide p-toluenesulfonyl hydrazone (16). On treatment of 16 with sodium ethoxide, the desired compound (3) was obtained. Reaction of 3 with p-nitrobenzoic acid gave the crystalline p-nitrobenzoyl ester. Treatment of uridine 5'-phosphate with 3 gave uridine 5'-(5-dimethylamino-2-oxidoisoquinolin-1-yl) methylphosphate (17). This labelled nucleotide was highly fluorescent, with an excitation maximum of 353 nm and an emission maximum at 523 nm. The fluorescence characteristics of 17 were compared with those of the model compound (13) and uridine 5'-(5-dimethylaminoisoquinolin-1-yl)methylphosphate (18).