The phenomenon of long term potentiation (LTP) of synaptic transmission, whereby a series of conditioning trains of impulses potentiate the size of synaptic potentials for periods in excess of hours, was discovered in the mammalian hippocampus by Lomo [1966, Acta Physiol. Scand. 68(Suppl. 277), 128] and subsequently characterized by Bliss and Lomo (1970, J. Physiol. 207, 61P). The search for the underlying mechanisms of LTP makes for fascinating reading. The induction of associative LTP was shown by Collingridge et al. (1982, J. Physiol. 334, 33-46) to be dependent on the presence of N-methyl-D-aspartate receptors, following the discovery of these receptors by Watkins and Evans (1981, A. Rev. Pharmac. Toxic. 21, 165-204). There has, however, been continuing controversy as to whether the maintenance phase of LTP over periods of hours may be attributed to an increase in the amount of transmitter released or to an increase in the number of glutamate receptors or both. There is more agreement on the important role or protein kinases in the maintenance phase of LTP. The role of LTP in memory is just now being elucidated.