This paper presents some of the general methodological and theoretical implications involved in psychophysiological, personality research--i.e., research attempting to determine the relationship between certain psychological characteristics and certain biological variables. Factors to consider in selecting both the experimental (psychological) variables and the physiological patterns to be studied are discussed. The concept of stress is briefly discussed and it is suggested that the variables be studied also when a psychological stressor is added to the subject. Some ways of exploring the interrelations among psychological and physiological variables are also discussed. These include direct observation of subjects, clinical interviews, case histories and questionnaires. It is also suggested that "a variety of tests that could perhaps be included in the field of expressive behavior should be used in research in this field." Studying "personal tempo" is suggested as one example of such research. It is concluded that "the list of both psychological and physiological variables to be investigated could be extended practically ad infinitum. The more adequate tests to be included in a particular research will depend on the problem to be investigated and on the interests and preferences of the investigator." Paper read at the Interamerican Society of Psychology, Santo Domingo, December 1953.