The extracellular concentrations of the amyloid precursor protein (APP) and neural-cell adhesion molecule (NCAM) in the dentate gyrus of the anaesthetized rat were assayed before and after the induction of long-term potentiation (LTP) in vivo. Levels of high molecular weight neurofilament protein and activity of the lysosomal enzyme arylsulphatase were measured as internal controls and indicators of neuronal damage. Ninety minutes after the induction of LTP, the concentrations of NCAM and APP increased, in an NMDA-dependent manner, in the absence of changes in neurofilament and arylsulphatase levels. The delayed changes in the extracellular concentration of these molecules may reflect events leading to morphological modifications following LTP.