The effect of prior lesioning of noradrenergic neurons with 6-hydroxydopamine (6-OHDA) on the ability of amitriptyline to enhance the responsiveness of hippocampal pyramidal neurons to serotonin (5-HT) was examined in Sprague-Dawley rats. In control rats, amitriptyline (5 mg/kg/day for 14 days), administered either by daily injection or by an osmotic minipump, enhanced the responsiveness of these neurons to microiontophoretically applied 5-HT, leaving their responsiveness to norepinephrine (NE) and to acetylcholine unaltered. However, in 6-OHDA-pretreated rats, amitriptyline failed to enhance the responsiveness of hippocampal pyramidal neurons to 5-HT. The effect of NE and, to a lesser extent, that of 5-HT were prolonged in 6-OHDA-pretreated rats, presumably because NE and 5-HT released by microiontophoresis are taken up by NE terminals in the intact rat. The present results provide additional evidence that the integrity of the NE system is required for antidepressant treatments to modify 5-HT neurotransmission.