Increased levels of somatostatin (SS) and neuropeptide Y (NPY) have been demonstrated in the hippocampal formation after kindling. The increase might be specifically associated with kindling, or be an effect of repeated seizures per se. In order to separate these two components we studied the effects of repeated electroconvulsive shocks (ECS) on hippocampal SS-like and NPY-like immunoreactivity and SS mRNA and NPY mRNA in situ hybridization. ECS elicit seizures without having a demonstrable kindling effect. Rats were subjected to 10, 20, or 36 ECS (50 mA, 0.5 s), given as one shock per day, 5 days per week. One, 2 and 30 days after the last ECS, the rats were killed, together with sham-treated control rats, and processed for immunocytochemistry and non-radioactive in situ hybridization. There was a bilateral increase in SS-like and NPY-like immunoreactivity 1 and 2 days after the last ECS in the outer part of the dentate molecular layer. This is the terminal field of the hilar SS-containing and NPY-containing neurons, which displayed both increased immunoreactivity and hybridization signal of the cell bodies. There was also a bilateral de novo expression of NPY-like immunoreactivity in the mossy fiber system, but this was not accompanied by the appearance of a detectable NPY hybridization signal over the parent dentate granule cell bodies. The increase in SS-like immunoreactivity and hybridization signal was most pronounced in the rats that had received the largest number of ECS. This was not observed for the NPY-like immunoreactivity and hybridization signal, where the increase appeared similar after 10, 20 and 36 ECS.(ABSTRACT TRUNCATED AT 250 WORDS)