Two experiments investigated phasic and tonic electrodermal responding to long, variable-duration stimuli in aversive conditioning procedures. Experiment I demonstrated reliable differential conditioning on both phasic (first interval response, FIR) and tonic (change in skin conductance level, delta SCL; spontaneous fluctuations) measures, using 10-40-s slides as conditioned stimuli (CSs) and electric shock as the unconditioned stimulus (UCS). Experiment 2 examined the sensitivity of phasic and tonic measures to UCS aversiveness. Both FIR and delta SCL were greater to a conditioned stimulus paired with shock than one paired with an innocuous reaction-time warning tone. Responding was reversed when the reinforcement contingencies were reversed. It was suggested that, with long duration CSs and an aversive UCS, phasic measures to stimulus onset, such as FIR, may reflect both stimulus significance and threat appraisal, whereas tonic measures reflect primarily arousal or anxiety arising from anticipation of the aversive unconditioned stimulus.