The importance of stress has long been recognized but confusions and vagueness of formulations have probably been responsible for the delay in submitting the effects of stress to systematic investigation. The discovery that stress, no matter how produced, always manifests itself in the form of a definite, stereotyped syndrome finally has made possible an 'operational definition'. The author considers only problems that may have interesting applications in psychiatry. He discusses, on the basis of personal experiments, 1, an 'operational definition' based on measurable indicators of stress; 2, steroid anesthesia; 3, the anticonvulsive and tranquilizing effect of steroids; 4, corticoids and muscular paralysis; 5, morphologic changes in the brain produced by corticoids; 6, stress and the inflammatory diseases; and 7, stress and sexual derangements.