Taltireline, a central nervous system activating agent, has two crystal forms (a-form and p-form). These crystal structures and the solvent-mediated transformation are described. The alpha-form was triclinic, P1, and the p-form was orthorhombic, P2(1) 2(1) 2(1). A conformational change associated with the transformation was also postulated. The solubility profile of polymorphs showed an enantiotropic system and the a-form chosen as a drug substance was found to be metastable at its crystallization temperature(10 degrees C). In order to suppress the transformation from the alpha form to the beta-form, the critical parameters that influence the crystallization process were examined. Experiments for measuring the transformation rate were carried out under various conditions using XRD, and the overall transformation rate constant k(T) and the waiting time a for the nucleation of the beta-form were estimated by fitting to an equation for the crystal growth rate. The results showed that the coexistence of MeOH, excess cooling during the crystallization, and fast stirring promoted the transformation. The desirable alpha-form can be consistently obtained by controlling these factors.