The purpose of the present study was to examine the role of serotonin (5HT) in the discriminative stimulus effects of kappa opioids. Pigeons were trained to discriminate 5.6 mg/kg of the kappa opioid, U50,488, from water. During substitution tests, both U50,488 and another kappa opioid, spiradoline, produced > 80% responding on the U50,488-appropriate key. In contrast, the non-opioid compound, phencyclidine and several serotonergic compounds failed to substitute for the U50,488 discriminative stimulus across a wide range of doses. During combination tests, the selective 5HT1A agonist, 8-OH-DPAT (0.001-3.2 mg/kg), dose-dependently attenuated the discriminative stimulus effects of 5.6 mg/kg U50,488 and 3.2 mg/kg spiradoline. This effect was reversed by the 5HT1A antagonist, NAN-190 (0.01-1 mg/kg), in a dose-dependent manner. Buspirone (0.01-10 mg/kg), a 5HT1A partial agonist, also attenuated the discriminative stimulus effects of the training dose of U50,488 but ipsapirone, another 5HT1A partial agonist, did not. Ketanserin, a 5HT2 antagonist, and MDL72222, a 5HT3 antagonist, attenuated the effects of U50,488, whereas the 5HT1B,1C agonist, mCPP, and the 5HT2 agonist, DOI, did not. Depletion of 5HT with p-CPA also attenuated U50,488's discriminative stimulus effects. Taken together, the results suggest that serotonin release is an important component in the discriminative stimulus effects produced by kappa opioids; however, the effects of DOI and mCPP alone suggest that activation of post-synaptic 5HT receptors is not sufficient to produce the full spectrum of kappa opioid discriminative stimulus effects.