The effects of chronic morphine administration on the rat cerebellum and silver stained nucleolus organizer regions in Purkinje cells were investigated by means of histological, histochemical and histometrical techniques. Thirty-two young (30-32 days of age) Wistar rats (equal numbers of both genders) were randomly divided into 2 groups, as control and morphine-treated, each having equal numbers of both genders, a total of 16 animals. The control animals were injected subcutaneously with 1mL/kg physiological saline, and the morphine-treated rats received 5 mg/kg morphine hydrochloride subcutaneously at daily intervals for 30 days. The thickness of the molecular and granular layers of the cerebellum, the diameters of the nuclei and nucleoli of cerebellar Purkinje neurones, and the number and size of the silver staining nucleolus organizer regions of the Purkinje cell nuclei were determined histomorphometrically. The morphine administration caused slight histological changes in the cerebellum. The molecular layer thickness of the cerebellum was significantly (P<0.05) reduced, and the decrease was mainly in the male animals. In contrast, the layer thickened insignificantly in the morphine-treated females. The granular layer thickened slightly but insignificantly in both genders of the morphine-treated group. The Purkinje cell count significantly decreased with morphine treatment. Nucleus size did not change with morphine treatment. However, morphine-treated animals had smaller nucleoli. It was concluded that morphine treatment caused significant histomorphological changes in the cerebellar cortex in a sexually dimorphic manner.