The effect of repeated treatment (5 and 10 mg/kg, po, twice daily, 14 days) with sertraline and citalopram (antidepressants which selectively inhibit the reuptake of 5-hydroxytryptamine (5-HT)) on the responsiveness of different 5-HT receptors to their agonists, was examined in rats and mice. Sertraline and citalopram (both at a dose 5 and 10 mg/kg) antagonized (the first one more potently) the hypothermia induced in mice by 8-OH-DPAT (a 5-HT1A agonist), but not the behavioural syndrome induced in rats by this substance. The m-chlorophenylpiperazine-induced hypothermia in mice (a 5-HT1B effect) was increased by sertraline and citalopram (only in a dose of 10 mg/kg). Both antidepressants, given repeatedly (as well acutely) attenuated exploratory hypoactivity induced in rats by m-chlorophenylpiperazine (a 5-HT1C effect). L-5-HTP-induced head twitches in mice (5-HT2 effect) were antagonized dose-dependently by both repeated sertraline and citalopram. Both antidepressants (citalopram only in higher dose) reduced the fenfluramine-induced hyperthermia in rats (5-HT2 effect). The results indicate that sertraline and citalopram given repeatedly decrease the responsiveness of 5-HT1A (presynaptic) and 5-HT2 receptors but increase the responsiveness of 5-HT1B receptors to respective agonists.