The reliability of classically conditioned skin conductance responses was investigated. Temporal stability was determined in 28 subjects studied three weeks apart (study 1), and internal consistency in 223 subjects studied once (study 2). A discriminative classical conditioning paradigm using slides with a duration of 8 s served as conditioned stimuli (CS) for an aversive unconditioned noise stimulus (study 1) or a mild unconditioned electric shock stimulus (UCS) (study 2). Electrodermal responses were recorded during a habituation phase (4 trials), an acquisition phase, where CS+ was paired repeatedly with the UCS, while CS- never was (8 trials), and an extinction phase during which shocks were withheld (8 trials each). First interval responses were measured 1-4 s after CS- onset during all phases of the experiment. During the acquisition and extinction phases, second interval responses were scored 5-9 s after CS- onset while third interval responses were determined 1-4 s after CS- termination. Internal consistency was significant for the first (rxy range 0.96-0.90), second (rxy range 0.84-0.54) and third (rxy range 0.96-0.86) skin conductance interval response. Temporal stability was highest for the first interval response (rxy range 0.72-0.37) and lowest for the second interval response (rxy range 0.51-0.05). It is concluded that the first interval skin conductance response shows adequate internal consistency and temporal stability to assess individual differences in classical conditioning.