Repeated electroconvulsive stimulations and other seizure modalities produce an increase in neuropeptide Y synthesis and local release in the rat hippocampus, and perhaps as a consequence, a change in the concentration of neuropeptide Y binding sites in the same region. The aim of the present study was to determine possible changes in the expression of neuropeptide Y receptor subtypes affected by repeated stimulations in the hippocampus. Rats were exposed to 14 daily stimulations, and the brains were removed 24h after the last stimulation. For in vitro receptor autoradiography and in situ hybridisation histochemistry, the brains were frozen, sectioned, and levels of neuropeptide Y binding sites and messenger RNA expressions were determined quantitatively on sections from the same animals. In order to determine the contribution of different neuropeptide Y receptor subtypes, serial sections were incubated with either 125I-labelled peptide YY alone or the same radio-labelled peptide mixed with an excess of a number of displacing compounds with affinity for either neuropeptide Y receptor subtype Y1, Y2, or both. Binding studies revealed that the majority of peptide YY binding sites was represented by Y2, and that electroconvulsive stimulations reduced the binding capacity or the concentration of this receptor. A prominent reduction of Y1-preferring binding sites was determined in the dentate gyrus, and to a lesser extent in the CA1 and CA3 regions. Similarly, the treatment produced a significant reduction of Y2-preferring binding sites in the CA1 and CA3 region, but not in the granular cell layer of the dentate gyrus. Using semi-quantitative in situ hybridization, Y1 receptor messenger RNA level in the granular cell layer of the dentate increased by the stimulations. In the same region, Y2 receptor messenger RNA was expressed in low to undetectable amounts, but after the repeated stimulations, this transcript was found in moderate to high levels. These data suggest that the neuropeptide Yergic system in the dentate gyrus and the pyramidal cell layer are affected by the treatment, and that this includes both Y1 and Y2 receptor subtypes. Because levels of messenger RNA and binding are distinctly regulated, the turnover of both Y1 and Y2 molecules is strongly increased under electroconvulsive stimulations, suggesting that the intrahippocampal neuropeptide Yergic neurotransmission is also increased under the stimulations.