The results presented in this article describe two distinct, Ca-regulated molecular pathways in presynaptic terminals and implicate these two pathways in differentially mediating neurotransmitter secretion and PTP. Our current view of the Ca-dependent triggering of secretion and PTP is shown in Fig. 9. According to this scheme, differential activation of these two pathways is achieved by a combination of diffusion-based dilution of Ca that enters the terminal through voltage-gated Ca channels and by coupling these pathways to Ca receptors with different affinities for Ca ions. A simple way to achieve these conditions is to position these two receptors at different distances from the Ca channels, as shown in Fig. 2. Given that Ca ions are involved in activating many different presynaptic processes (Fig. 1), we propose that closer scrutiny of the molecular physiology of nerve terminals will reveal a wide variety of Ca-activated pathways responsible for producing these diverse processes.