We investigated the effects of pethidine on neurons and the relationship between neuronal apoptosis and pethidine dependence. Thirty male Sprague-Dawley rats were randomly divided into control, dependence and withdrawal group. The animals were treated with saline or pethidine by subcutaneous injection for 60 days. Dose of pethidine was from 20 to 140 mg kg(-1)d(-1). Spontaneous withdrawal was induced by ceasing pethidine administration in withdrawal group. Ultrastructure was observed by transmission electron microscope. The ncNOS, caspase-3 and Bax IHC were performed on paraffin sections by SP method. TUNEL was used as a marker for identification of neuronal apoptosis in cerebral cortex and periaqueductal grey matter. Under ultrastructure, there were many features of neuronal apoptosis and necrosis. There also were many TUNEL staining positive neurons scattered in brain tissue. The grey scale of IHC staining and number of positive neurons of ncNOS, caspase-3 and Bax increased distinctly in chronic dependence and withdrawal (P<0.05). Pethidine resulted in neuronal apoptosis, degeneration and necrosis. These damages could be pathological basis of pethidine dependence and tolerance.