LH-RH analogs cause some inhibition of growth of pancreatic cancers. Syrian golden hamsters bearing chemically induced pancreatic cancers were treated with [D-Trp6]LH-RH for 3 d before sacrifice. LH-RH receptors were localized by electron-microscopic immunohistochemistry in the tumor cells of both treated and untreated hamsters. [D-Trp6]LH-RH treatment resulted in a marked increase in the concentration of LH-RH receptors in the nuclei. The dissociation constants (Kd) and the maximal binding capacity of the LH-RH receptors (Bmax), measured by radioreceptor assay, were higher in the nuclei of the pancreatic tumor cells of hamsters treated with [D-Trp6]LH-RH than in the untreated controls. Pancreatic cells of tumor-free hamsters did not show immunostaining for LH-RH receptors. A possible correlation between the increase in the concentration of the LH-RH receptors in the nuclei and the tumor growth-inhibiting activity of [D-Trp6]LH-RH is suggested.