Structural changes induced by high frequency (HF) stimulation on granule cells of the hippocampal dentate gyrus of rats with inborn high (HP) and low (LP) learning capacity was examined in this study. Two types of axodendritic synapses were processed morphometrically; the axonal bouton on dendritic shafts and those making contact through dendritic spines. The occurrence of granule cell spine synapses on LP rats before tetanus is less than the one in control animals. However, after long-term potentiation (LTP) produced by HF stimulation the number of spine synapses decreased, as happened in the control rats. Moreover, the number of granule cells shaft synapses counted on LP rats, before tetanus is less than the one on control rats, and after LTP this number increased greatly. There was no change in the number of granule cell shaft synapses in HP rats before and after LTP. LP rats which did not show LTP in spite of HF stimulation did not show changes in the number of spine and shaft synapses. From these results we can conclude that: (1) the great hippocampal synaptic plasticity observed in HP rats compared to LP rats can be attributed, at least in part, to the increased number of spine and shaft synapses observed in HP rats; (2) the minor number of spine and shaft synapses in LP rats may account for the impossibility to obtain LTP after HF stimulation in these animals. © 1990.