1. Phase I clinical trials are usually carried out in healthy volunteers. In addition to economic gain, factors that may influence willingness to participate include scientific interest, curiosity and choice for risky activities. 2. We assessed the relationship between personality variables and volunteering for clinical pharmacology research. Two personality questionnaires, the Sensation Seeking Scale (SSS, form V) and the Eysenck Personality Questionnaire (EPQ), were administered to 48 male healthy university students who volunteered to participate in a phase I clinical trial and to 43 male university students who were not willing to participate in phase I clinical trials. General norm data were also used for the comparison of results. 3. When healthy volunteers were compared with unwilling subjects, significant differences were found in thrill-and-adventure seeking (7.9 vs 6.7, P = 0.0034), experience seeking (6.4 vs 5.2, P = 0.0012), disinhibition (6.2 vs 4.3, P < 0.0001), boredom susceptibility (3.9 vs 2.8, P = 0.0073), total sensation seeking trait (24.3 vs 19.0, P < 0.0001), extraversion (15.1 vs 13.3, P = 0.0490), and psychoticism (4.4 vs 3.5, P = 0.0086). When healthy volunteers were compared with general norm data similar statistically significant differences were found in all these scales, except for boredom susceptibility and psychoticism. 4. The personality profile of healthy volunteers was characterized by a higher sensation seeking trait and extraversion as compared with individuals who were not willing to participate in phase I clinical trials and general norm data.