This study examined phasic and tonic measures of electrodermal activity in a simple habituation paradigm with innocuous visual stimuli. Prestimulus skin conductance levels were taken as measures of the arousal level existing at each stimulus presentation, and the subsequent electrodermal responses were taken as indices of the phasic orienting reflex (OR) elicited by each stimulus. The arousal level was considered as a tonic OR measure. Both the tonic and phasic ORs showed effects of trials. Arousal levels also showed an initial sensitization, as suggested by dual-process theory. After within-subject effects of arousal changes over trials were removed by simple regression analyses, novelty still remained as the major determinant of phasic OR magnitude. This finding is discussed in relation to the OR mechanism. © 1993.