A review of the literature on PCP reveals that voluntary intoxication by taking this drug can and does impair the mental process in many cases. The doctor testifying as an expert witness in a diminished capacity case must have a basic understanding of the following legal principles: (1) the presumed innocence of the accused and the State burden of proving each element of the offense charged beyond a reasonable doubt; and (2) proof of the existence of a 'union or joint operation of act and intent, or criminal negligence.' The test of diminished capacity, now adopted by the California courts, holds that, in general, evidence of diminished mental capacity, whether caused by disease, drugs, intoxication, or trauma, may show that the accused did not have a specific intent or the specific mental state which is an element of the charged offense. The expert witness should understand the difference between legal insanity and diminished capacity. Furthermore, it is important that the defense lawyer and the psychiatrist cooperate in presenting the issue of diminished capacity and the defense lawyer should describe the accused's life history in a sympathetic way when using the diminished capacity defense. Five references and a note are included.