Schizophrenia is often characterized by successive periods of psychotic symptoms and remission. Phenomenological and be- havioral changes sometimes occur within a few days or weeks prior to the appearance of psychotic symptoms (Herz and Melville 1980; Subotnik and Nuechterlein 1988). If further developed, these "prodromal signs" might serve as early warning signs of impending psychotic episodes and, with appropriate intervention, might be used to circumvent subsequent relapses; however, phenomenological and behavioral prodromal signs require accurate self-monitoring and reporting of subtle experi- ential and behavioral changes, and may have limited sensitivity and specificity (see review by Norman and Malla 1995). Physiological measures that do not rely on self-report might be useful supplemental prodromal signs. A promising measure in this context is electrodermal activity (EDA), which is a measure of palmar sweat gland activity and a peripheral index of sympathetic nervous system arousal (Dawson et al 1990). EDA abnormalities in schizophrenia have been noted for many years (see reviews by Bernstein et al 1982; Ohman 1981). More relevant to the study of prodromal signs, however, is the finding that increases in EDA occur in schizophrenia patients during a psychotic episode compared to a symptomatically remitted pe- riod (Dawson et al 1994), and in the weeks prior to psychotic relapse or exacerbation in 2 patients tested on a weekly basis (Dawson et al 1992, pp 305-306). These data suggest that increased EDA may serve as a prodromal sign in at least some schizophrenia patients. To test this hypothesis, we examined EDA measures in a symptom-free 3-week period prior to a psychotic relapse or exacerbation, a time period in which prodromal signs might be expected to appear. We then compared the EDA obtained from this "prodromal period" with the same EDA measures obtained in a "control period" that was not shortly follow