The behaviorally defined analgesia evoked by direct application of norepinephrine (NE) or serotonin (5-HT) into the rat spinal subarachnoid space was assessed 1 to 28 days after respective intrathecal injection of 6-hydroxydopamine or 5,6-dihydroxytryptamine. These pretreatments effected supersensitivity to intrathecally administered NE and 5-HT, respectively, observable as early as 24 hr after neurotoxin-induced lesion. Dose-response curves for both NE or 5-HT, determined 7 days after pretreatment with their respective neurotoxins, were shifted significantly to the left as compared to controls. The onset of supersensitivity correlated well with the depletion of neurotransmitter levels in the spinal cord in the case of NE, but much correlation was not found for 5-HT. Whereas monoamine supersensitivity was undetectable 28 days postlesion, there was no significant recovery of spinal cord monoamine content at this time. Neurotoxin pretreatment was found to effect a significantly greater potentiation of NE- or 5-HT-induced analgesia than presynaptic reuptake blockade and elicited supersensitivity to agonists not readily taken up by presynaptic terminals, suggesting the involvement of postsynaptic components. Rats pretreated 7 days before with 5,6-dihydroxytryptamine exhibited some supersensitivity to intrathecal NE. Supersensitivity to 5-HT was not observed after 6-hydroxydopamine pretreatment. Rats made tachyphylactic to the antinociceptive effect of intrathecally injected NE displayed a complete lack of analgetic response to 5-HT. Similarly, rats tachyphylactic to 5-HT exhibited a subnormal analgetic response to NE. Combination pretreatment with both neurotoxins produced significant hyperalgesia as compared to yoked controls and to nociceptive thresholds determined before neurotoxin administration.