We have examined basal phosphoinositide turnover in synaptosomes obtained from the dentate gyrus of anaesthetized rats in which long-term potentiation was induced unilaterally in perforant path-granule cell synapses. Relative to the unpotentiated side. [3H]myo-inositol labelling of inositol phosphates was significantly enhanced 45 min and 3 h after induction of long-term potentiation, but reduced after 2.5 min. Similarly, [14C]arachidonic acid labelling of 1,2-diacylglycerol was increased 45 min and 3 h after induction of long-term potentiation, but reduced after 2.5 min. In a second series of experiments, induction of long-term potentiation was blocked by stimulation of the commissural projection to granule cells. In synaptosomes prepared from this tissue, there was no difference in phosphoinositide turnover between tetanized and control sides at any of the three post-tetanic intervals. We conclude that in the dentate gyrus, long-term potentiation is associated with an increase in phosphoinositide turnover which is established between 2.5 min and 45 min post-tetanus and which persists for at least 3 h.