A commonly used method to remove polysynaptic components of test PSPs is to elevate action potential threshold of interneurons with high extracellular concentrations of divalent cations ('Hi-Di'). Extrapolation to normal conditions requires that Hi-Di have negligible effects on synaptic transmission. We examined effects of Hi-Di on EPSPs from sensory neurons (SNs) onto motor neurons (MNs) of Aplysia in the pleural-pedal and abdominal ganglia, and in dissociated cell culture. In ganglia, standard Hi-Di solutions eliminated spontaneous input from interneurons as well as polysynaptic components of PSPs evoked by single action potentials in single SNs, but failed to block polysynaptic PSPs evoked by nerve stimulation. Hi-Di solutions had no effect on activity-dependent synaptic depression or posttetanic potentiation, or facilitation by serotonin (5-HT). Unexpectedly, standard Hi-Di solutions substantially reduced sensorimotor EPSPs in all preparations, whereas a solution containing 2.2x[Ca(2+)] and 2x[Mg(2+)] blocked the polysynaptic component of EPSPs without obvious changes to the monosynaptic component. In contrast to previous observations in Aplysia, and to predictions of the (J. Physiol. 193 (1967) 419) model, tripling the normal extracellular concentrations of Ca(2+) and Mg(2+) failed to increase sensorimotor EPSPs. Depression of EPSPs by these Hi-Di solutions may result from reduced spike invasion into presynaptic terminals.