Systemic, intra-raphe and microiontophoretic administration of the 5-hydroxytryptamine (5-HT)1C/5-HT2 agonist (1-(2,5-dimethoxy-4-iodophenyl)-2- aminopropane (DOI) inhibited the firing of 5-HT neurones in the dorsal raphe. DOI administered systemically and directly into the raphe also decreased the extracellular concentration of 5-hydroxytryptamine (5-HT) in the frontal cortex. In contrast, the administration of DOI directly into the frontal cortex did not significantly alter the concentration of frontal cortical extracellular 5-HT. The reduction of the firing rate of 5-HT neurons in the dorsal raphe and extracellular 5-HT concentration in the frontal cortex induced by systemic administration of DOI could not be blocked by the 5-HT2 antagonist ketanserin, ritanserin (5-HT2/5-HT1C antagonist) or the putative 5-HT1A antagonist, pindolol. These results suggest that the inhibition of 5-HT neuronal firing seen with administration of DOI is mediated via an action within the dorsal raphe and at least in close proximity to the 5-HT neurone cell bodies. The decrease in frontal cortical extracellular concentration of 5-HT release was not due to a direct action in the frontal cortex itself and may possibly be as a result of the decrease in the firing rate of the 5-HT neurones in the dorsal raphe. The mechanism of action of DOI to produce these effects is, however, unclear and warrants further investigation.