A complete study of basic hydrolysis of the pyrazolidinone ring by ab initio calculations at RHF/6-31+G*// RHF/6-31+G* and MP2/6-31+G*//MP2/6-31+G* has been carried out. The alkaline hydrolysis has been studied through a B-AC2 mechanism, characterized by a nucleophilic attack of the hydroxyl group on the carbonyl of the gamma-lactam ring, formation of the tetrahedral intermediate, and cleavage of the C2N3 bond to yield the final reaction product. In the gas phase, the interaction of OH- with the carbonyl carbon to form a tetrahedral intermediate takes place without any barrier height. Two possible mechanisms have been considered for the transfer of the hydroxyl hydrogen to the nitrogen of the gamma-lactam : a stepwise mechanism involving the cleavage of the C2N3 bond and subsequent transfer of the hydrogen to the gamma-lactam nitrogen and st concerted mechanism. The MP2/6-31+G*//MP2/6-31+G* barrier heights are 32.72 and 25.64 kcal/mol, respectively. The elimination reaction, which in the gas phase may interfere with the nucleophilic attack, has also been studied.