The effects of intracerebroventricular neuropeptide Y (NPY) or somatostatin were studied upon hippocampal EEG seizures elicited by electrical stimulation of the rat dentate gyrus or subiculum. At doses of 6 and 12 nmol, the latter dose being more effective, NPY reduced the primary afterdischarge duration (1.ADD) and almost completely abolished the secondary afterdischarge. The reduction in 1.ADD resulted from an increase in afterdischarge threshold. The reduction in secondary afterdischarge duration was independent of a reduction in 1.ADD. This implies that NPY not only exerts antiepileptiform effects in the dentate gyrus and subiculum, but also in areas to which epileptiform EEG activity spreads before reverberating. In addition, NPY strongly reduced seizure-related 'wet dog shakes' (WDS). This is consistent with previous studies showing that the dentate gyrus is essential for the generation of WDS. However, NPY inhibited WDS even when 1.ADDs were evoked which did not differ from those of vehicle rats, indicating extra-dentate inhibition by NPY as well. No effects were seen with somatostatin. These results show that NPY exerts antiepileptiform effects in vivo, suggesting that increased NPY in the hippocampal formation observed after seizures is a compensatory anti-seizure response.