The proton nuclear magnetic resonance spectra of a reaction mixture of hydrazine and ethyl acetoacetate (EAA) in aqueous solution under flowing and stopped-flow conditions provides kinetic information concerning the rapid addition step to form the carbinolamine (CA) intermediate and the slower dehydration and cyclization steps to form the anti hydrazone and 3-methylpyrazol-5-one, respectively. The addition step was studied while flowing at rates faster than the dehydration and cyclization rates so that no product signal was observed. Under these conditions, a broad signal resulting from the coalescence of the CH3 proton resonances of EAA and CA is observed. Analysis of the line shape of this broad signal yields the addition rate constant, which has a value that appears to be pH dependent and is of the order of 103 M−1 s−1. Stopped-flow spectra provide the time dependence for the reactant and product signals. Analysis of these time dependences provides information concerning the mechanism of product formation. In contrast with hydroxylamine, CA appears to undergo dehydration and cyclization at comparable rates in the pH range, 8.8–9.2 when hydrazine is the nucleophile. In addition, the anti hydrazone cyclizes at an appreciable rate. These intramolecular cyclization rates are compared with the intermolecular values.