1. Although placebo administration is now commonly used as a control condition during clinical pharmacology studies conducted in healthy volunteers, data in placebo-treated subjects usually receive little attention. 2. The profile of several physiological and hormonal parameters was reviewed during the placebo sessions of five double-blind phase I studies involving hospitalisation of healthy volunteers for 2 weeks or more. 3. A clear trend towards an increase in heart rate, which culminated at about 10 beats min-1 at the end of the placebo period was observed, whereas both systolic and diastolic blood pressures remained globally unchanged. 4. Increases in the time to sleep initiation or in asthenia self-ratings during the placebo sessions suggested poor neuropsychiatric tolerability of experimental conditions in some of the studies. 5. In conclusion, all these data confirm that subjects change during the course of these studies. This is an important reason for conducting phase I studies under double-blind placebo-controlled conditions and to refrain from within-group comparisons (vs baseline, first-day effect).