It is widely believed that a rise in post-synaptic calcium concentration ([Ca2+]i) is a necessary step in the induction of long-term potentiation (LTP) (Bliss and Collingridge, Nature, 361 (1993) 31-39). In this experiment, we examine the involvement of voltage-gated Ca2+ channels (VGCC) in the induction of AP5-sensitive LTP induced by theta-burst stimulation in guinea-pig hippocampal CA1 neurons. The VGCC blockers, Ni2+ (25 microM, T-channel blocker) or omega-AgaIVA (60 nM, P-channel blocker), which have no effect on synaptic transmission, suppress 60% or 78% of the theta-burst induced LTP, respectively. This implies that Ca2+ entry through VGCC is an important step in this form of LTP.