ABSTRACTBased on a large screening project, 24 electrodermal nonresponders to nonsignal tones were chosen from an undergraduate population and compared on multiple electrodermal, heart rate, and personality measures to an equal number of randomly selected control subjects. Questions addressed in this study included the reliability of the nonresponder category with repeated testing, its stability across situations, its concurrent relationship with other psychophysiological measures, and its ability to predict scores on more standardized measures of individual differences.It was determined that nonresponders continued to produce smaller specific skin conductance responses (SCRs) during a second testing session two weeks after the initial screening, and these differences remained when subjects were tested under signal conditions. However, 50% of the initial nonresponders did produce SCRs by the completion of all testing. Nonresponders and controls differed on other electrodermal measures as well, with nonresponders producing fewer spontaneous fluctuations under all conditions and lower skin conductance levels during initial screening and under signal conditions. Nonresponders and control subjects also differed in heart rate response during the signal procedure. Control subjects produced the anticipated cardiac deceleration, whereas nonresponders produced a pronounced heart rate increase.The two groups did not differ on measured personality characteristics, but when nonresponders were divided into subgroups based on consistency of response pattern, several differences did emerge.