The prototropic tautomerism of 3-methyl-5-pyrazolone was studied byusing Density Functional Theory along with 1H, 13Cand 15N-NMR. The results show that for the isolated moleculeand low polarity solvent the 5PYR-1 tautomer, a keto form, is the predominant species. In DMSO solution the predominant tautomeric form is 5PYR3, a hydroxytautomeric species. For almost every atom an excellent agreement was foundbetween the experimental and calculated NMR spectra using GIAO method at DFT/B3LYP/6-311+G (2p,d) level