This study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the effects of water stress on grain filling. Two high lodging-resistant rice (Oryza sativa L.) cultivars were pot-grown. Three treatments, well-watered, moderate water-stressed (MD), and severe water-stressed (SD), were imposed from 9 d post-anthesis until maturity. Grain filling rate and grain weight were significantly increased under MD but decreased under SD. The two cultivars behaved the same. ABA concentration in the grains was very low during the grain filling stage, reaching a maximum when the grain filling rate was highest. Both the grain filling rate and ABA concentration were substantially enhanced by water stress. In contrast to ABA, concentrations of ethylene and 1-aminocylopropane -1-carboxylic acid (ACC) in the grains were very high at early grain filling stage and sharply decreased during the linear period of grain growth. MD reduced, whereas SD remarkably increased, their accumulation. The ratio of ABA to ACC was increased in MD grains but decreased in SD grains, indicating that there was a greater enhancement of ABA concentration than ethylene production in the MD treatment only. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at the early grain filling stage significantly increased grain filling rate. Spraying with ethephon (ethylene-releasing agent) or fluridone (inhibitor of ABA synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate the grain filling rate, and a high ratio of ABA to ethylene enhances grain filling rate.