Burger's two polymorphs of tolbutamide (TB), an oral hypoglycemic agent, were obtained by spray-drying the drug dissolved in a mixed solvent of ethanol/dichloromethane (Form IV) and allowing Form IV to stand at constant temperatures and humidities (Form II). These polymorphs were characterized by various physical methods [e.g., powder X-ray diffractometry, differential scanning calorimetry, infrared spectrometry, and solid-state carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy] and compared with two other TB polymorphs Forms I and III. The 13C NMR spectra showed that the chemical shift and the peak shape of resonance associated with the toluene and n-butyl moieties of TB were different for each of the four polymorphs, whereas the carbonyl carbon was unchanged, indicating different conformations and molecular motions of the toluene and n-butyl moieties in the solid states. Form IV converted itself to Form II within 3 h when it was stored at 45 degrees C and 75% relative humidity (RH) and, in turn, Form II transformed to Form I at higher temperatures. The conversion of Form IV to Form II proceeded according to a zero-order equation (Polany-Winger equation), and that of Form II to Form I according to a first-order equation. The increase in RH accelerated the polymorphic transition of Form IV. Both the apparent dissolution rate and the solubility of Form IV were nearly identical with those of Form II, because the former changed to the latter during the dissolution, but their dissolution rates and solubility were higher than those of Forms I and III. These dissolution characteristics of TB polymorphs were reflected in the oral absorption behavior in dogs; that is, the bioavailability increased in the order Form I < Form III < Form II approximately Form IV.