The objectives of this study were to-investigate ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) in rice grains and root bleeding sap during the grain filling period and their relationship to the grain filling rate.Two high lodging-resistant rice (Oryza sativa L.) cultivars were grown in pots or tanks.Three treatments,including well watered (WW),moderate soil-drying (MD) and severe soil-drying (SD),were conducted from 9 days of post-anthesis until maturity.The effects of chemical regulators on the concentrations of ethylene and ACC in the grains were also studied.The results show that MD significantly increased the grainfilling rate and grain weight,whereas SD significantly reduced the grain-filling rate and grain weight.Concentrations of ethylene and ACC in the grains were very high at the early grain filling stage and then sharply decreased during the linear period of grain growth.MD reduced the ACC concentrations and ethylene evolution rate,whereas SD remarkably increased the ACC concentrations and ethylene evolution rate.Both the ethylene evolution rate in rice grains and the ACC concentrations in the root-bleeding sap were significantly and positively correlated with the ACC concentrations in rice grains.The ethylene evolution rate was significantly and negatively correlated with the grain-filling rate.The application of amino-ethoxyvinylglycine (AVG),an inhibitor of ethylene synthesis,at 9-13 days of postanthesis significantly reduced the ACC concentrations and ethylene evolution rate of grains,but significantly enhanced the activities of sucrose synthase,ADP glucose pyrophosphorylase and soluble starch synthase.The results were reversed when ethephon,an ethylenereleasing agent,was applied.The results suggest that moderate soil drying during the grain-filling period in rice could inhibit the production of ethylene and ACC and therefore accelerate grain filling and increase grain weight.