The aim of the present study was to quantify the personality structure and motivation of medical students, who are often the most readily available source of healthy volunteers, since it has long been known that the pharmacodynamics and pharmacokinetics of drugs can be substantially modified by predominant personality traits (e.g. neuroticism, extraversion). Over the course of 4 years, a total of 337 subjects (165 males, 172 females) out of the 496 medical students asked, participated in the study after appropriate instruction. Students were tested using the Motivation Q-Sort method which, by means of questions ("questionnaire sort"), investigates whether a subject tends to react in a success-motivated (SM) or a failure-motivated (FM) mode. The variable measured is the so-called net hope (NH), where NH > 1.6 corresponds to SM and NH < -0.3 corresponds to FM. We also used the Freiburg Personality Inventory (FPI) as a suitable method of determining personality structure. The predominant traits of interest found were nervousness (FPI 1), extraversion (FPI E), and neuroticism (FPI N). In the first series of tests (primary selection), motivation only was determined in 337 volunteers. The range was fairly broad, with NH values from 4.32 (highly SM) to -3.09 (highly FM). In the second series of tests, about 60 SM and 60 FM subjects were selected. The Motivation Q-Sort method was repeated with students placed under more difficult conditions, and the FPI was also performed.(ABSTRACT TRUNCATED AT 250 WORDS)