Abstract: AimTo study the influence of light and heat on the stability of procaine hydrochloride injection. MethodsAccelerated tests upon exposure to light at high temperatures were employed. ResultsIn experiments with either isothermal heating or exposure to light at high temperatures, the drug degradation rate obeys first-order kinetics. The total rate constant, k_total, caused by both light and heat can be divided into two parts: k_total=k_dark+k_light, where k_dark and k_light are the rate constants caused by heat and light, respectively. The k_light can be expressed as k_light=A_light·E·exp-E_a,lightRT. Where E is the illuminance of light, A_light is an experimental constant related to light sources, and E_a,light is an experimental constant. ConclusionBecause the form of k_light is similar to the Arrhenius equation, it is suggested that E_a,light might be the observed activation energy of the rate-determining step of the subsequent processes of the photochemical reaction. This viewpoint is supported by the fact that the E_a,light is independent of light sources.