Summary When palmar skin potential level (SPL) is recorded duringperiods of minimal arousal sweating, and a concentration of potassium chloride within physiological limits is used for the external electrolyte, the value of the "basal" SPL (BSPL) recorded can be expressed as a function of the external/epidermal potassium concentration gradient.The present study reports comparison of BSPL values obtained with the use of physiologically comparable concentrations of sodium and potassium chlorides. The results suggest the possibility that the BSPL methodology may provide a means of monitoring electrolyte shifts in the human subject.In addition, sex differences in BSPL values were demonstrated.Palmar skin potential level (SPL) is one of the electrodermal measures that can be recorded from human subjects and used to assess their psychophysiological state (VENABLES and MARTIN, 1967a); human SPL is usually negative at a palmar surface electrode with reference to an "inactive" electrode site on an ipsilateral, abraded, forearm surface. The electrodes used may be of a silver/silver chloride (Ag/AgCl) chamber type, with sodium or potassium chloride (NaCl, KCl) as the external electrolyte at the two electrode/epidermal interfaces (VENABLES and SAY-ER, 1963). The negativity of SPL decreases with decreasing arousal (LEIDERMAN and SHAPIRO, 1964;CHRISTIE, 1970), a condition in which palmar sweating is reduced (MARTIN and VENABLES, 1966) if ambient temperatures are moderate (NEUMANN, 1968, p. 110). The absolute value of SPL has, however, been reported to be lowered when the concentration of the external electrolyte is reduced (VENA-BLES and SAYER, 1963;ROTHMAN, 1954;EDELBERG, 1963).Examination of the physiological mechanisms underlying electrodermal phenomena has suggested that sudorific factors may be significantly implicated in the generation of SPL (MARTIN and VENABLES, 1966): VENABLES and MARTIN