Long-term forest fire retardants for fire prevention purposes are currently used, or are under consideration, in many parts of the world. Their use requires, among other things, knowledge about weathering of retardants with time, which may lead to the need for re-application. Rainfall is a factor that can lead to retardant depletion from the fuels. In this study, the rate of depletion was evaluated experimentally using Aleppo pine (Pinus halepensis) needles. The needles were made into small bundles and immersed in retardant, which was FIRE-TROL 936 concentrate, diluted to 20% (v/v) with water. The retardant-treated needles, after drying, were exposed to natural rain in three different rainfall events, and retardant depletion was measured. A regression equation was developed with percentage retardant removal as the dependent variable. The natural logarithm of rainfall quantity, expressed in mm of rain, and the duration of rain, expressed in min, were the two independent variables. A simple equation can be used to support pre-suppression planning and to assess environmental effects.