Since the clinical response to haloperidol varies from individual to individual, an investigation was carried out to investigate some of the physiochemical properties of haloperidol which may affect its pharmacological activity. The stability of haloperidol in aqueous solutions containing 1% lactic acid, with or without 0.1% methyl paraben, was investigated. A colorimetric method based on a reaction between methyl orange and haloperidol was used. The samples (0.5 mg/ml) were stored in glass and plastic containers at 25, 37, 50, 80 and 110 degrees C for 9 months. The resulting data were analysed using principles of chemical kinetics. The shelf-life of haloperidol solutions was estimated to be at least 4 years. Other physiochemical factors which can affect the pharmacological activity of haloperidol were investigated, namely its solubility and complex-forming properties.