In an article previously published in this Review I demonstrated that power-seeking, by which I mean the pursuit of social dominance, has a biochemical marker, namely, whole blood serotonin (WBS). Those individuals who are especially hard-charging and competitive have especially high WBS levels. This paper presents the results from an investigation of dynamics in the response of high WBS individuals to actual competition. My expectation—that they would exhibit special physiological activation in the face of challenge—is fully borne out by the evidence. Four hormonal indicators of activation were followed in blood samples taken during a series of social competitions. All four show distinctive patterns for the high WBS group. These results, in addition to providing new evidence on the behavior of the endocrine system in competitive settings, impressively support the view that WBS is a biological property having fundamental significance for behavioral political science.