Abstract Relative potencies of external Ca 2+, Sr 2+ and Ba 2+ to trigger GABAergic synaptic transmission were evaluated by applying the patch-clamp technique to both presynaptic and postsynaptic hippocampal neurons prepared from neonatal rats. Action potentials were evoked by application of voltage pulses to presynaptic neurons, and GABAergic synaptic currents were recorded in voltage-clamped postsynaptic neurons. No stimuli were delivered during replacement with test solutions and only five pulses were applied to the presynaptic neuron in each test solution. During the five-pulse application, the amplitude of synaptic currents was constant in Ca 2+-containing solutions, but decreased successively in Ba 2+- and Sr 2+-containing solutions without Ca 2+. Thus, the amplitude of synaptic currents induced by the first pulse in each ionic condition was used to evaluate the potency of divalent cations. The lowest external concentration required to trigger the transmission was 0.3 mM for Ca 2+, 1 mM for Sr 2+ and 2 mM for Ba 2+, and the concentration required to achieve the same effect as with 2 mM Ca 2+ was 6 mM for Sr 2+ and 10 mM for Ba 2+. These results strongly suggest that Ba 2+ as well as Sr 2+ can be substituted for Ca 2+ in GABAergic synaptic transmission and the order of potency is Ca 2+ > Sr 2+ > Ba 2+.