Combined toxicity of herbicides to non-target crops is usually resulted from their successive application.The present study was conducted to assess the combined toxicity of flufenacet(FLU)and imazaquin(IMA)to sorghum with their concentration in soil pore water.The concentrations that inhibited growth by 50%(IC50)of FLU and IMA individually and their combination estimated from the herbicide concentrations in soil pore water notably differed from those based on the amended concentrations,due to the decline in bioavailability resulting from adsorption of the herbicides onto soil.According to the amended concentrations,the combined effect of FLU and IMA in soil on sorghum growth was identified as additive action.Based on the concentration in soil pore water,however,it was determined to be antagonism,which was identical to that observed in a test using culture solution.The results revealed that pore water herbicide concentration might be an effective tool to assess the combined toxicity of herbicides in soil to rotational crops. Combined toxicity of herbicides to non-target crops is usually induced from their successive application. The present study was conducted to assess the combined toxicity of flufenacet (FLU) and imazaquin (IMA) to sorghum with their concentration in soil pore water. inhibited growth by 50% (IC50) of FLU and IMA individually and their combination estimated from the herbicide concentrations in soil pore water notably differed from those based on the amended concentrations, due to the decline in bioavailability resulting from adsorption of the herbicides onto soil. According to the amended concentrations, the combined effect of FLU and IMA in soil on sorghum growth was identified as additive action. Based on the concentration in soil pore water, however, it was determined to be antagonism, which was identical to that observed in a test using culture solution. The results revealed that pore water herbicide concentration might be an effective tool to assess the combined toxicity of herbicides in soil to rotational crops.