Long-term potentiation (LTP) is characterized by a long-lasting enhancement of synaptic efficacy which may be due to an increase in synaptic numbers. The present study was designed to verify the validity of this suggestion using recently developed unbiased methods for synapse quantitation. LTP was elicited in young adult rats by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 h after the fourth stimulation. Stimulated but not potentiated and implanted but not stimulated rats served as controls. Synapses were examined in the middle (MML) and inner (IML) molecular layer of the hippocampal dentate gyrus. Using the stereological disector technique, unbiased estimates of the number of synapses per neuron were differentially obtained for the following morphological synaptic types: axodendritic synapses involving dendritic shafts, non-perforated axospinous synapses exhibiting a continuous postsynaptic density (PSD) and perforated axospinous synapses distinguished by a fenestrated, horseshoe-shaped or segmented PSD. A major finding of this study is that the induction of LTP is accompanied by a selective increase in the number of synapses with segmented PSDs. This change was detected only in the potentiated synaptic field (MML), but not in an immediately adjacent one (IML) which was not directly stimulated during the induction of LTP. It is strongly suggested by the latter finding that the increase in the number of axospinous synapses exhibiting segmented PSDs is associated with LTP. Such a highly selective modification of connectivity, which involves only one particular subtype of synapses in the potentiated synaptic field, is likely to represent a structural substrate of the enduring augmentation of synaptic efficacy typical of LTP.