Long-term potentiation(LTP) can be induced by several ways. We have reported that the perfusion of ATP induces LTP (Fujii, S. et al. Neurosci. Lett. 187, 130-132, 1995). In the present study, we have measured the change of [Ca 2 + ] i in CA1 neurons of guinea pig hippocampal slices with fluorometry in relation to ATP induced LTP. The perfusion of ATP induced [Ca 2 + ] i increase mainly at the pyramidal cell layer and the stratum oriens, but relatively a little at the stratum radiatum.