1. An intracerebral perfusion method, brain microdialysis, was used to assess changes of 5-hydroxytryptamine (5-HT) release in the ventral hippocampus of the chloral hydrate-anaesthetized rat in response to systemic administration of a variety of 5-HT1 receptor agonists. 2. A stable output of reliably detectable endogenous 5-HT was measured in dialysates collected from ventral hippocampus with the 5-HT reuptake inhibitor, citalopram, present in the perfusion medium. 3. Under these conditions the putative 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) caused a dose-dependent (5-250 micrograms kg-1, s.c.) reduction of 5-HT in hippocampal dialysates. 4. Similarly, the putative 5-HT1A agonists gepirone (5 mg kg-1, s.c.), ipsapirone (5 mg kg-1, s.c.) and buspirone (5 mg kg-1, s.c.) markedly reduced levels of 5-HT in hippocampal perfusates whereas their common metabolite 1-(2-pyrimidinyl) piperazine (5 mg kg-1, s.c.), which does not bind to central 5-HT1A recognition sites, had no effect. 5. 5-Methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), a drug with reported high affinity for brain 5-HT1B binding sites, also produced a dose-dependent (0.25-5 mg kg-1, s.c.) decrease of hippocampal 5-HT output. 6. These data are direct biochemical evidence that systemically administered putative 5-HT1A and 5-HT1B agonists markedly inhibit 5-HT release in rat ventral hippocampus in vivo.