This study aimed to investigate the changes occurring in estrogen receptor ( ER) alpha-positive cells, proliferating cells, apoptotic cells and malondialdehyde ( MDA) expression in the pancreas of experimentally induced adult diabetic rats and to determine the effect of orally administered lycopene on these changes. Experimental diabetes was induced using a single dose of 50 mg/kg streptozotocin ( STZ). Following the administration of STZ, four groups of animals were established: Control1corn oil, control1lycopene, diabetic1corn oil and diabetic1lycopene. The expressions of ER a, Ki-67, and MDA were determined through immunohistochemistry in sections taken from pancreas tissue samples at the end of the experiment. Apoptotic cells were determined through the TUNEL method. In the diabetic groups, the densities of ER a expression in islets and ER alpha-positive cells in exocrine parts increased. Whereas the number of proliferating Ki-67 positive cells was higher in the diabetic groups, no significant difference was observed in terms of apoptotic cell number between the control and diabetic groups. Lycopene in general did not have any effect on any of the parameters analyzed in the study. The presence of ER alpha-positive cells around the islets was demonstrated for the first time in diabetic groups. Based on these observations, demonstrating the antioxidant effects of lycopene in the pancreas of diabetic rats may be possible by increasing the dose and/or the duration of lycopene. Anat Rec, 300:2000-2007, 2017. (C) 2017 Wiley Periodicals, Inc.