Taltirelin, a central nervous system activating agent, has two crystal forms (alpha- and beta-forms). The solvent effect on the crystallization behavior of the polymorphs was investigated in a mixture of water and methanol (MeOH) under agitated crystallization. At a low concentration of MeOH (0 and 10 wt%), the alpha-form is precipitated at the initial stage of crystallization regardless of the seed form. Thereafter the alpha-form is transformed into the beta-form. In 30% MeOH, only the beta-form is precipitated regardless of the seed form. In 30% MeOH without agitation, the beta-form grew on the surface of the alpha-seed crystal. The effect of MeOH on the crystal growth rate was also examined at 10 degrees C and at a prescribed supersaturation ratio. As the MeOH concentration increased, the crystal growth rate of the alpha-form decreased and that of the beta-form inversely increased. These results are discussed with regard to the conformer of taltirelin in solution.