New 4-arylhydrazono-3-methoxymethyl-2-pyrazolin-5-ones (5a-j) and 4-arylhydrazono-3-methoxymethyl-1-phenyl-2-pyrazolin-5-ones (6a-j) were synthesized by the reaction of ten novel methyl 2-arylhydrazono-4-methoxy-3-oxobutanoates (4a-j) with hydrazine hydrate and phenylhydrazine, respectively. Treatment of 5a with acetic anhydride yielded 1-acetyl-4arylhydrazono-3-methoxymethyl-2-pyrazolin-5-one (7). The structures of the tautomerically dynamic compounds were established by spectral data (IR, H-1-NMR, C-13-NMR, EIMS and CIMS) and elemental analysis. The analgesic activity of the title compounds was determined by the modified Koster's test. The most active compound was 4-[(4-chlorophenyl)hydrazono]-3-methoxymethyl-1-phenyl-2-pyrazolin-5-one (6c) demonstrating twice the activity (60%) exerted by the reference drug acetylsalicylic acid (ASA, 27%). The majority of the tested compounds were found to be more effective than ASA.