This article addresses the serotonin hypothesis of depression, as it was formulated in clinical and laboratory experiments during the 1950s. In the first instance I argue that the `challenge' posed by patients' subjectivities in clinical investigations into the potentially anti-depressant drug iproniazid was not solely due to the tensions generated by the subject/object dichotomy, but to an excess that exceeds the properties of the objects of the experiment, as well as its requirements and conditions. I then suggest that the serotonin (...) hypothesis too is possessed of an excess, and that this can be understood in terms of a real (actual and virtual) existence in duration. By exploiting the notions of the event and of duration, I offer an under-standing of the hypothesis that pertains not to reduction, reproducibility and sameness, but to differentiation, innovation and creation. (shrink)