The influence o f different levels o f peat and amount and frequency o f simulated rain on the mobility of alachlor and terbuthylazine in soil was investigated under greenhouse conditions. Movement o f the two herbicides down the soil profile, assessed by bioassay using Cucumis sativus, was inversely related to the organic matter content o f the soil, but the amount and frequency o f rain directly influenced the extent o f leaching o f the herbicides. In general, terbuthylazine was less mobile than alachlor in the two soils studied, namely Serdang and Selangor series. The phytotoxic levels o f terbuthylazine were restricted to the 0 - 15 cm zone in both soils when 640 ml simulated rain was applied over 16 days. Alachlor, on the other hand, moved to depths o f 20 and 30 cm in Selangor and Serdang series, respectively, when soil columns were exposed to the same watering treatment.