It has recently been suggested that central 5-HT1A autoreceptors are already desensitised after single-dose 5-HT1A agonist treatment. In turn, this would lead to an attenuated feedback suppression of transmitter release from 5-HT neurones, and thus to enhanced 5-HT synaptic transmission. In the present study in vivo brain microdialysis techniques were used in an attempt to test this hypothesis. The results show that single-dose pretreatment with the reference 5-HT1A receptor agonist 8-hydroxy-2-(din-propylamino)tetralin, 8-OH-DPAT, (i) did not significantly alter the baseline output of 5-HT in the rat ventral hippocampus 24 h later, and (ii) did not alter the release-reducing response to 5-HT1A agonist (8-OH-DPAT, ipsapirone or BMY 7378) challenge under the same conditions. These observations indicate that the functional responsiveness of the 5-HT release-controlling 5-HT1A autoreceptors is maintained after bolus 8-OH-DPAT pretreatment. When related to the acute 8-OH-DPAT-induced reduction in raphe 5-HT1A radioligand binding density recently reported by others, the present results are consistent with a large functional overcapacity of this 5-HT1A receptor population. The mechanism by which 5-HT1A receptor-mediated hypothermia and hyperphagia are rapidly attenuated by a previous large single dose of a 5-HT1A receptor agonist remains to be explained.