Whether or not the psychiatrist testifies on the ultimate issue in insanity defense cases, it is critically important that he familiarize himself with the applicable legal standards and interpretations in order properly to relate his clinical findings to the relevant criteria for insanity and thereby enhance the probative value of his testimony. This is the third in a series of articles which attempts to explicate judicial and statutory standards of insanity and correlate them with the psychiatrist's findings of psychopathology. This article analyzes the Model Penal Code formulation of insanity, with special emphasis on the all important distinction between "know" and "appreciate." This formulation permits the defendant possessed of mere surface knowledge or cognition to be exculpated, requiring that he have a deeper affective appreciation of the legal and moral import of the conduct involved if he is to be held criminally responsible. The Model Penal Code approach more readily lends itself to application as a standard of responsibility in cases involving affective disorders. An important disorder within this group, postpartum depression, is discussed in the context of raising the insanity defense in a case of infanticide.