The membrane potential of identified nerve (Retzius) cells and neuropil glial cells from 11 (±1) day-old embryos of the leechHirudo medicinalis was recorded using conventional intracellular microelectrodes. At this stage all ganglia of the segmental nervous system are formed. The membrane potential of Retzius cells was −68±4 mV (±SD,n=8), and showed a slope of 42 mV between 10 mM and 100 mM external K concentration. Retzius cells were able to fire action potentials which had a fast Na-dependent component, and, under appropriate conditions, also generated slow Ca (Ba) action potentials. The mean membrane potential of the neuropil glial cell at physiological K concentration (4 mM) was −83±5 mV (±SD,n=10), and showed a dependence of 56 mV for a tenfold change in the external K concentration (> 4mM). Neuropil glial cells showed no signs of voltage-activated excitability, but they repeatedly depolarized in the presence of 0.1 mM 5-HT.