Levels of melatonin in rabbit eye tissues were detected by radioimmunoassay. Solutions of met-enkephalin, leu-enkephalin, α-endorphin and β-endorphin were given topically. Met-Enkephalin and α-endorphin lowered levels of melatonin in the iris, iris root-ciliary body, retina and choroid; leu-enkephalin raised levels in the retina and lowered them in other tissues. β-Endorphin only lowered levels in the iris root-ciliary body. DAGO (a μ agonist) given i.v. lowered levels of melatonin in the iris, iris root-ciliary body and retina. The δ and σ agonists given i.v. only lowered levels in the iris root-ciliary body, and a k agonist given i.v. raised levels in the ciliary body. No opiate binding sites could be detected in the rabbit iris or iris root-ciliay body for any class of receptor. Our data suggest opioids may be useful for treating glaucoma.