Objectives: The aim of this study was to investigate the effects of pinealectomy and melatonin treatment on the rat thymus gland characteristics, taking into consideration possible gender differences. Materials and methods: Thirty adult Wistar rats of both sexes were divided into three groups. Group C and group PX served as control groups and included sham-pinealectomized and pinealectomized animals that were treated with 10% ethanol solution (0,1ml/daily, subcutaneous). Animals from third group (group PXM) underwent pinealectomy and seven days after surgery started receiving melatonin dissolved in 10% ethanol solution (3mg/kg/daily, subcutaneous). All animals were treated for 4 weeks. Results: Volume density of the thymus cortex showed statistically significant (p
rat, pineal gland, thymus gland, melatonin.