The cardinal feature of multiple personality disorder (MPD) is the existence of two or more alter personality states that exchange control over the behaviour of an individual. Numerous clinical reports suggest that these alter personality states exhibit distinct physiological differences. We investigated differential autonomic nervous system (ANS) activity across nine subjects with MPD and five controls, who produced "alter" personality states by simulation and by hypnosis or deep relaxation. Eight of the nine MPD subjects consistently manifested physiologically distinct alter personality states. Three of the five controls were also produced physiologically distinct states, but these differed from those of the MPD subjects. A habituation paradigm demonstrated carryover effects at the ANS levels from one state to the next for both groups.