Effects of environmental factors on bioactivity, adsorption, and persistence of pendimethalin was studied in the laboratory and greenhouse using rice (Oryza sativa L.) as the bioassay species. Herbicide bioactivity was inversely correlated with the organic matter (OM) content of the soil and directly correlated with herbicide concentrations. Pendimethalin adsorption increased proportionately at higher content of soil organic matter (SOM) and with increasing herbicide concentration. The apparent half-life of pendimethalin decreased from 23.9 days at 25°C to 19.4 days at 35°C in Selangor series soil and from 24.4 days at 25°C to 18.5 days at 35°C in Serdang Series soil. The apparent half-life of pendimethalin was shorter at a moisture level of 70% gravimetric water content than at moisture level of 30% in both soils. Under field conditions, the apparent half-life was 6.3 and 12.3 days at application rates of 1 kg and 3 kg ha-1, respectively.