The dose-effectiveness of pancuronium as it relates to membrane potentials, action potentials, electrical membrane constants, miniature endplate potentials, endplate potentials, and quantal release was studied in murine phrenic nerve—diaphragm preparations invitro. Emphasis was placed on comparison of presynaptic with postsynaptic effects of pancuronium under similar experimental conditions. At low concentrations of pancuronium (5±10 g/ml or less), no presynaptic effect was found. At high concentration (5±10–7g/ml), pancuronium depressed quantal release to 26 per cent of control in cut-fiber preparations and 40 per cent of control in high-magnesium preparations. Postsynaptic effects, as measured by the amplitude of miniature endplate potentials and relative depolarization induced by 20 μM carbachol, revealed depression to 16 and 22 per cent of control, respectively, at a pancuronium concentration of 5±10–7g/ml. Pancuronium had no effect on directly clicked action potentials and electrical membrane constants. The authors conclude that presynaptic as well as postsynaptic effects of pancuronium in paralytic doses are essential in contributing to the total efficacy of neuromuscular depression.