The effect of 5-hydroxytryptamine on the induction of long-term potentiation by a train of high frequency pulses (100 Hz; 1 s) or by a stimulation consisting of one burst of five pulses at 100 Hz delivered 170 ms after a single pulse (primed burst) was investigated in the CA1 region of the rat hippocampal slicein vitro with extracellular recordings. Superfusion with 5-hydroxytryptamine (3–30 μM) produced a concentration-dependent decrease in amplitude of the population spikes evoked by test stimuli. The presence of 5-hydroxytryptamine (30 μ M) did not affect the magnitude of long-term potentiation produced by the high-frequency stimulation but it prevented the long-term potentiation induced by a primed burst. The action of 5-hydroxytryptamine was mimicked by the 5-hydroxytryptamine1A agonist 5-carboxamidotryptamine (0.3 μM) and blocked by the 5-hydroxytryptamine2/5-hydroxytryptamine1a antagonist spiperone (3 μM) or by the 5-hydroxytryptamine1/5-hydroxytryptamine2 antagonist methiothepin (1–10 μM). The selective 5-hydroxytryptamine2 antagonist ritanserin (1 μM) did not antagonize the block of long-term potentiation produced by 5-hydroxytryptamine. The selective 5-hydroxytryptamine3 antagonists (3-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205–930; 1 nM) and ondansetron (GR-38032; 30 nM) did not affect the reduction in the population spike produced by application of 5-hydroxytryptamine. In contrast, a primed burst delivered at the fifth minute of 5-hydroxytryptamine application in the presence of a 5-hydroxytryptamine3 antagonist induced a long-term potentiation. It is concluded that activation of 5-hydroxytryptamine1a and 5-hydroxytryptamine3 receptors blocked the induction of long-term potentiation induced by primed burst stimulation but not that induced by a l s high-frequency train. Given the heterogeneous localization of these receptor subtypes, it is suggested that the overall action of 5-hydroxytryptamine was exerted by hyperpolarizing pyramidal cells via 5-hydroxytryptamine1a receptors and by increasing spontaneous discharges of GABAergic interneurons via stimulation of 5-hydroxytryptamine3 receptors.