The present study was undertaken to investigate the nature of the effect of pertussis toxin on the responsiveness of two potentially distinct subgroups of postsynaptic serotonin 1A (5-HT 1A) receptors of rat hippocampus CA3 pyramidal neurons: those located at the level of the cell body, which can be activated by microiontophoretically-applied 5-HT 1A receptor agonists, and those located on dendrites, which can be activated by endogenous serotonin released by the stimulation of the ascending serotoninergic pathway. The former receptors (denoted as extrasynaptic) have been previously demonstrated to be sensitive to pertussis toxin, whereas the latter (denoted as intrasynaptic) have been shown to be pertussis toxin-insensitive. Rats treated with the 5-HT 1A receptor agonists flesinoxan or BMY 42568 were used to determine whether tonic activation of extrasynaptic 5-HT 1A receptors would prevent their inactivation by pertussis toxin. A pretreatment with p-chlorophenylalanine was used to determine whether a serotonin depletion would render the intrasynaptic 5-HT 1A receptors sensitive to pertussis toxin. The responsiveness of CA3 pyramidal neurons to the suppressant effects of microiontophoretically-applied serotonin, 8-hydroxy-2-(di-n-propylamin)-tetralin, baclofen and GABA or to endogenously-released serotonin, elicited by the stimulation of the ascending serotoninergic pathway, was studied one to 10 days after the intrahippocampal injection of pertussis toxin. When compared to control saline-treated rats, the treatments with flesinoxan (5 mg/kg/day, s.c.) and BMY 42568 (5 mg/kg/day, s.c.) delivered for 14 days by osmotic minipumps, starting three days prior to the injection of pertussis toxin, significantly attenuated the effect of pertussis toxin on the responsiveness of CA3 pyramidal neurons to microiontophoretic applications of serotonin and 8-hydroxy-2-(di-n-propylamino)-tetralin, as well as baclofen, an agonist of GABA B receptors, which share the same G proteins with 5-HT 1A receptors. The two-day pretreatment with p-chlorophenylalanine (350 mg/kg/day, i.p.) did not render the intrasynaptic 5-HT 1A receptors sensitive to pertussis toxin, as indicated by the unchanged efficacy of the stimulation of the ascending serotonin pathway in the suppressing the firing activity of CA3 dorsal hippocampus pyramidal neurons. Our results suggest that the sustained activation of extrasynaptic 5-HT 1A receptors prevents the pertussis toxin-induced ADP ribosylation of G protein α subunit, and thereby protects an amount of G proteins sufficient to maintain the function, not only of 5-HT 1A, but also of GABA B receptors. The observation that the intrasynaptic 5-HT 1A receptors remained pertussis toxin-insensitive, despite lowering of endogenous serotonin content, further suggests that these 5-HT 1A receptors may be functionally distinct from extrasynaptic 5-HT 1A receptors.