A theoretical quantum mechanical analysis of the structural, conformational and reactivity properties of the schistosomicidal drug niridazole is presented. The computations are carried out in the CNDO/2 approximation including full molecular geometry optimization and modellization of the solvent effect. The change in molecular properties when the molecule approaches a macromolecular receptor is simulated with decreasing values for the dielectric constant of the medium. In addition, the reactivity properties of niridazole are analyzed within the frontier orbital approximation. The structural analysis of the optimized geometries reveals a planar structure for the nitro group and the thiazole ring whereas the imidazolidinone ring is found to be non planar. The conformational analysis shows that the flexibility of the molecule, measured by the barriers to rotation of the imidazolidinone ring against the thiazole ring, increases greatly when the molecule approaches to a macromolecular receptor. Our reactivity results are in agreement with the experimental data and they support the hypothesis that the metabolic activation of niridazole in the schistosome involves a nucleophilic attack. The nitro group and some selected positions on the thiazole ring are the preferred points for this attack.