Tests of performance emphasizing alertness, psychomotor speed, and reaction time are widely used in testing the effect of beta-blockers on the central nervous system. However, this aspect of brain function is only one of many that are amenable to psychometry. This paper is presented from the stand-point of psychiatry and provides an overview of psychometric testing that highlights the points of contact with general medicine and psychiatry, and with research in both these areas. The applications of psychometry in defining relatively stable individual differences such as intelligence or personality, and in distinguishing morbid from normal states, are discussed. The use of psychometry in detecting performance changes relevant to pharmacological research is reviewed. Interpretation of all test results depends upon an understanding of the structure of the test itself. The confidence that may be placed on interpreting results depends on adequate reliability. For any inference to be drawn there must be both normative data and criterion validity, i.e. discovery of the score point at which there are demonstrable changes in day-to-day function other than those detected within the confines of the test. Unfortunately many psychometric tests fall short on these data and, therefore, inferences from their use should be made cautiously.