The functions of oxytocin in the female are thought to be confined to the processes of milk ejection and parturition. In this study the role of oxytocin in the body's response to stressful stimuli has been examined. Physical immobilization of rats and forced swimming caused large increases in the secretion of the hormone, whereas vasopressin levels remained unchanged. These findings classify oxytocin, in contrast to vasopressin, as a so-called 'stress hormone' and suggest its importance in processes other than those related to reproduction.