It has been established that multiple types of Ca2+ channels participate in triggering neurotransmitter release at central synapses, but there is uncertainty about the nature of their combined actions. We investigated synaptic transmission at CA3-CA1 synapses of rat hippocampal slices and asked whether the dependence on omega-CTx-GVIA- sensitive N-type channels and omega-Aga-IVA-sensitive P/Q-type Ca2+ channels can be altered by physiological mechanisms. The reliance on multiple types of Ca2+ channels was not absolute but depended strongly on the amount of Ca2+ influx through individual channels, which was manipulated by prolonging the presynaptic action potential with the K+ channel blocker 4-aminopyridine (4-AP) and by varying the extracellular Ca2+ concentration ([Ca2+]o). We quantified the influence of spike broadening on Ca2+ influx through various Ca2+ channels by imposing mock action potentials on voltage-clamped cerebellar granule neurons. In field recordings of the EPSP in hippocampal slices, action potential prolongation increased the EPSP slope by 2-fold and decreased its reliance on either N-type or P/Q-type Ca2+ channels. The inhibition of synaptic transmission by N-type channel blockade was virtually eliminated in the presence of 4-AP, but it could be restored by lowering [Ca2+]o. These results rule out a scenario in which a significant fraction of presynaptic terminals rely solely on N-type channels to trigger transmission. The change in sensitivity to the neurotoxins with 4-AP could be explained in terms of a nonlinear relationship between Ca2+ entry and synaptic strength, which rises steeply at low [Ca2+]o, but approaches saturation at high [Ca2+]o. This relationship was evaluated experimentally by varying [CA2+]o in the absence and presence of 4-AP. One consequence of this relationship is that down-modulation of presynaptic Ca2+ channels by various modulators would increase the relative impact of spike broadening greatly.