This single EPSP does not sufficiently depolarize the membrane to generate an action potential.]]
[[Image:Synapse diag5.png|thumb|300px|The summation of these three EPSPs generates an action potential.]]
In [[neuroscience]], an '''excitatory postsynaptic potential''' ('''EPSP''') is a temporary depolarization of postsynaptic [[membrane potential]] caused by the flow of positively charged [[ion]]s into the postsynaptic cell as a result of opening of [[ligand-gated ion channel]]s. They are the opposite of [[inhibitory postsynaptic potential]]s (IPSPs), which usually result from the flow of ''negative'' ions into the cell or positive ions ''out'' of the cell. A [[postsynaptic potential]] is defined as excitatory if it makes the neuron more likely to fire an [[action potential]]. EPSPs can also result from a decrease in outgoing positive charges, while IPSPs are sometimes caused by an increase in positive charge outflow. The flow of ions that causes an EPSP is an '''excitatory postsynaptic current''' (EPSC).
EPSPs, like IPSPs, are graded (i.e. they have an additive effect). When multiple EPSPs occur on a single patch of postsynaptic membrane, their combined effect is the sum of the individual EPSPs. Larger EPSPs result in greater membrane depolarization and thus increase the likelihood that the postsynaptic cell reaches the threshold for firing an [[action potential]].