BACKGROUND Systemically administered opioids induce analgesia in part by spinal noradrenergic, serotonergic, and cholinergic mechanisms. The current study tested whether antinociception from systemically administered opioids could therefore be enhanced by intrathecal injection of a monoamine reuptake inhibitor to potentiate the action of spinally released norepinephrine and serotonin (amitriptyline) and intrathecal injection of a cholinesterase inhibitor to potentiate the action of spinally released acetylcholine (neostigmine). METHODS Rats were prepared with chronic lumbar intrathecal and femoral intravenous catheters and nociceptive threshold was assessed by hind paw withdrawal to a radiant heat stimulus. An isobolographic design was used to distinguish between additive and synergistic interactions. RESULTS Intravenous morphine and intrathecal neostigmine, but not intrathecal amitriptyline, caused dose-dependent antinociception alone. Combining any two of these three treatments yielded a synergistic interaction compared to each alone, whereas combining all three yielded an additive interaction compared to each two-way interaction. Intrathecal amitriptyline did not affect antinociception from intrathecal clonidine or intrathecal carbamylcholine. CONCLUSIONS These data suggest that intrathecal doses of amitriptyline resulting in potentiation of intravenous morphine antinociception may not be adequate to block muscarinic receptors, because they did not affect carbamylcholine-induced antinociception. These results further support the relevance of spinal monoamine reuptake and cholinesterase inhibition to synergistically enhance analgesia from systemic opioids.