1. Cultured bull-frog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. The cells were superfused with a nominally calcium-free Ringer solution containing tetrodotoxin (3 microM), magnesium (10 mM), cobalt (1 mM), barium (2 mM), 4-aminopyridine (3 mM) and caesium (2