Animals under stress take adaptive actions that may lead to various types of behavioral disinhibition. Such behavioral disinhibition, when expressed excessively and impulsively, can result in harm in individuals and cause a problem in our society. We now show that, under social or environmental stress, mice deficient in prostaglandin E receptor subtype EP1 (Ptger1 ؊/؊ ) manifest behavioral disinhibition, including impulsive aggression with defective social interaction, impaired cliff avoidance, and an exaggerated acoustic startle response. This phenotype was reproduced in wild-type mice by administration of an EP1-selective antagonist, whereas administration of an EP1-selective agonist suppressed electric-shockinduced impulsive aggression. Dopamine turnover in the frontal cortex and striatum was increased in Ptger1 ؊/؊ mice, and administration of dopaminergic antagonists corrected their behavioral phenotype. These results suggest that prostaglandin E2 acts through EP1 to control impulsive behavior under stress, a finding potentially exploitable for development of drugs that attenuate impulsive behavior in humans.dopamine ͉ aggression ͉ behavioral disinhibition A ll organisms are repeatedly disturbed or threatened by changes in the internal or external environment. Such a state of perturbed homeostasis is referred to as ''stress'' (1, 2). Stress results not only from physical stimuli, such as heat, injury, or disease but also from psychological stimuli, such as exposure to a novel environment or a predator. Stress manifests itself in various characteristic responses that include emotions such as fear and aggression and activation of the neuroendocrine and sympathetic nervous systems (3). These stress responses are thought to be initiated and regulated in the CNS by biochemical processes that are evoked by threatening stimuli (''stressors'') and that determine the activity of several distinct but interacting neuronal pathways (4). However, the neural mechanisms responsible for initiation and regulation of stress responses have remained largely unknown. Neither is it known whether different forms of stressors (for example, physical vs. psychological stimuli) mobilize the same or distinct mechanisms to initiate and regulate their stress responses. Given that the stress responses are elicited to promote adaptation, severe dysfunction of either initiation or regulation mechanisms can not only be a threat to survival, but can also result in various physiological, psychological, or even psychiatric disorders (3, 5).We have been studying the roles of prostaglandins (PGs) in various physiological and pathophysiological processes. PGs are arachidonic acid metabolites formed by the sequential actions of cyclooxygenase (COX) and respective synthases, and they include PGD 2 , PGE 2 , PGF 2␣ , PGI 2 , and thromboxane A 2 . PGs interact with eight types or subtypes of G protein-coupled receptors, including the PGD receptor, four subtypes of PGE receptor (EP1, EP2, EP3, and EP4, which are encoded respectively by the genes Ptger1...