The isometric responses of isolated human posterior ciliary artery to adrenergic agonists, histamine (HIS), and 5-hydroxytryptamine (5-HT) were studied in passively stretched ring segments mounted in a myograph bath. Cumulative dose response curves were measured for nine agonists: HIS, 5-HT, dopamine (DOPA), epinephrine (A), norepinephrine (NA), tyramine (TYR), phenylephrine (PHE), isoproterenol (ISOP), and xylazine (XYL), and the log(molar concentration) at which one half of the maximum active tension was developed (EC50) was estimated. The ring segments were unresponsive to DOPA and XYL; HIS and ISOP produced biphasic responses with a mild relaxation for low concentrations and small contractions for high concentrations of the agonist. The remaining agonists caused contractile responses of magnitude listed in the rank order following compared with the maximum active tension in response to 0.124 M K(+)-Krebs: Kmax much greater than A greater than 5-HT = PHE greater than NA greater than TYR It was concluded that functional HIS, alpha 1-adrenergic, and 5-HT receptors were present on human posterior ciliary artery but that there are no alpha 2-adrenergic receptors.