Luteinizing hormone-releasinghormone (LH-RH), a water soluble decapeptide, was found to be so firmly bound to a subcellular particulate structure in rat hypothalamus that only negligible amounts were solubilized during the homogenization and centrifugation processes. The structures binding LH-RH were localized in the 1.2 Msucrose portion of a discontinuous sucrose gradient. Another neurohormone, vasopressin, was more dispersed and located in the heavier portion of the gradient. Cholinesterase activity was also widely distributed throughout the gradient and its distribution was not closely related to the LH-RH section of the gradient.