The current study advances a biosocial model of negotiation, in which the effects of estradiol and opponent gender on competitive behavior are examined. Sixty-four female participants engaged in a computer-mediated negotiation simulation and completed measures assessing psychological distance, negotiation goals, opponent perceptions, and self-presentation concerns. Results demonstrated that psychological distance, estradiol, and opponent gender interact to predict competitive and conciliatory negotiation behavior. This study carries substantial implications for conflict management theory and practice as it illustrates the joint influence of biological and social situational factors on negotiation behavior.