The effect of temperature and relative humidity (RH) on the stability of imidapril hydrochloride (IMD) in solid state was investigated. The main aim of this study was to determine the most appropriate conditions of storage and manufacture of IMD so that the efficiency of the technological process could be improved and its costs could be minimized. A reversed-phase high-performance liquid chromatography was validated and applied for the determination of IMD degradation samples under the following operating conditions: stationary phase, LiChrospher 100 RP-18 (size 5 μm) 250 × 4 mm I.D., and mobile phase, acetonitrile-methanol-phosphate buffer, pH 2.0, 0.035 mol L(-1) (60:10:30 v/v/v). The effect of temperature on IMD degradation rate was analyzed under increased RH ≈ 76.4% (within temperature range of 70-90°C) and decreased RH ≈ 0% (within temperature range of 90-110°C). The influence of RH was investigated under 90°C within RH range of 25.0-76.4%. IMD degradation accords with autocatalytic reaction model, and RH has no influence on its mechanism yet it increases its rate. The reaction also accelerates under high temperatures and in the presence of IMD degradation product. Pure IMD is more stable than other structurally related angiotensin-converting enzyme inhibitors, such as enalapril maleate, but it still should be stored in tightly closed containers and protected from moisture and high temperatures.