The purpose of this study was to determine the mating system of the lesser rice-field rat as well as the effects of quinestrol treatment (10mg/kg) on their behaviour.Pair-choice preference tests and dyadic encounters were performed with the behavioral interactions between same sex pairs and opposite sex pairs recorded using a video collection system.Males (n=11) and females (n=11) were treated by gavage for 3 days.No significant difference was found in pair-choice preference tests by females toward strange versus familiar males,which suggests that the mating system of R.losea is not monogamous.Female rats did not show significant difference in odor or individual preference tests for untreated male rats.At one week after treatment,female rats preferred quinestrol-treated males,which implied that the mating chances of male rats did not decline after quinestrol administration.Male rats treated with quinestrol exhibited no change in their ability to recognize female rat odor,which showed that quinestrol did not change the courtship behavior of male rats in the short term.Through repeated testing,we determined that quinestrol administration disturbed the dominant–subordinate relationship of males immediately after administration.However,the dominant–subordinate relationship stabilized after 1 to 2 weeks.R.losea is not monogamous; quinestrol treatment may increase male rat aggression,and female rats preferred quinestrol-treated males,which could suggest that quinestrol-treated males can compete with untreated males for mates and increase the efficiency of fertility control.