In the present study, we investigated the pharmacological characteristics of electrically stimulated [3H]-serotonin release from mammalian iris-ciliary bodies. Isolated bovine and human iris-ciliary bodies were loaded with [3H]-serotonin, superfused with Krebs buffer solution and then stimulated with trains of 300 direct current (d.c.) pulses to initiate the release of the transmitter. The modification of this [3H]-serotonin release process by various serotonergic agonists and antagonists was studied in order to define the pharmacology of serotonin receptor(s) present in the iris-ciliary body. In bovine iris-ciliary body, electrically-evoked [3H]-serotonin release was calcium-dependent, tetrodotoxin-sensitive and was enhanced by serotonin (EC50=200n M) and 5-carboxmidotryptamine (EC50=4n M). The rank order of potency of agonists in enhancing field-stimulated [3H]-serotonin release was: 5-carboamidotryptamine>m-chlorophenylbiguanide>2-methyl-5-hydroxytryptamine=5-methoxy-dimethyltryptamine>serotonin>5-methoxy-tryptamine>L-694,247=α-methyl-5-hydroxytryptamine>CGS 12066A=8-hydroxy-2-(di- n -propylamino)tetraline. Serotonin and m-chlorophenylbiguanide also enhanced electrically-evoked [3H]-serotonin release from human iris-ciliary bodies with EC50s of 3μM and 30n M, respectively. The pharmacological profile displayed by serotonin receptor agonists was supported by the potent antagonism of the serotonin-induced enhancement of [3H]-serotonin release by 5HT7receptor antagonists SB-258718 (IC50=18.6±1.2n M;n=4) and mesulergine (IC50=0.26±0.05n M;n=4). However, antagonists at 5HT6and 5HT3receptors exhibited a relatively weak blockade of serotonin induced enhancement of field-stimulated [3H]-serotonin release. These studies have shown the presence of functionally active prejunctional 5HT7autoreceptors regulating the release of [3H]-serotonin from bovine iris-ciliary bodies. Excitatory prejunctional 5-HT autoreceptors also exist in human iris-ciliary bodies. It is possible that these serotonin autoreceptors may have relevance to the regulation of aqueous humor dynamics in the anterior uvea.