The reaction of the indoline alkaloid 12-hydroxynorfluorocurarine with phenylhydrazine produced 1,2-seco-12-hydroxynorfluorocurarine-2,17-phenylhydrazone, the structure of which was established by an X-ray crystal structure analysis. The proposed mechanism of formation of N was confirmed by quantum-mechanical calculations. The reaction was shown to proceed via a Michael mechanism with phenylhydrazine N atoms adding to C2 and C17 of the alkaloid to form a pyrazole ring and cleave the indole N1-C2 bond. Quantum-chemical calculations using the product from the reaction of 12-hydroxynonrfluorocurarine and phenylhydrazine as an example explained the proposed mechanism.