The polymorphic behaviour of nifedipine was investigated using differential scanning calorimetry (DSC), modulated temperature DSC (MTDSC) and hot stage microscopy (HSM). DSC and MTDSC scans of glassy nifedipine samples revealed the presence of several heating rate-dependent events occurring prior to the main melting endotherm (172 degreesC) including solid-solid recrystallisation on MTDSC curves. These events were related to the recrystallisation and transformation between different polymorphs of nifedipine. Recrystallisation, solid-solid and solid-liquid-solid transformation processes occurred in glassy nifedipine when heated from 45 degreesC to its final melting point of 172 degreesC, including a glass transition at similar to48 degreesC. It was concluded that there were probably four polymorphic forms of nifedipine. The chemical stability of nifedipine during DSC scanning was established by high performance liquid chromatography (HPLC) with diode array UV detection.