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The effects of intrahippocampally administered catechol- and indoleamines on the two types of hippocampal seizure discharges elicited by electrical and chemical stimulation were examined in unanesthetized rabbits. The catecholamines norepinephrine (NE) and dopamine (DA), injected into the hippocampus in doses of 100--200 micrograms, inhibited electrically induced hippocampal seizure discharges with a 50% increase in the stimulation threshold. However 5-hydroxytryptamine (5-HT) at doses of 50 micrograms to 100 micrograms caused no effect on electrically induced seizure discharges. On the contrary, 5-HT and 5-hydroxytryptophan (5-HTP) at a dose of 50 micrograms potentiated carbachol (5 micrograms)-induced hippocampal seizure discharges, prolonging the duration of seizure discharge three times the control. NE and DA had no effect on this chemically induced hippocampal discharges. It is therefore suggested that the effects of monoamines on hippocampal seizure discharges are very much dependent on the type of stimulation employed for the induction of this phenomenon.