Morphological and histological studies of the adrenal gland of the adult White Leghorn chicken (Gallus domesticus) were performed. Macro- and microscopic observations of azan- or formal-dichromate stained (to observe the chromaffin reaction of adrenal medulla) sections were conducted. The right and left adrenal glands differed in shape, position, weight, length, width and thickness. The adrenal glands were encapsulated with a thin connective tissue containing blood vessels. The adrenal parenchyma was composed of two main tissue types, the cortex and medulla; these were mixed throughout the organ. The adrenal gland could be divided into a subcapsular layer, peripheral zone, and central zone. The whole subcapsular layer was occupied by medulla, and the proportion of medulla in the central zone (49.7%) was larger than that in the peripheral zone (24.8%). Approximately 60% of the adrenal gland was cortex, 39% was medulla, and the remaining 1% was sinusoids. The adrenal cortico-medullary ratio in the adult chicken was approximately 1.6:1. Cortical cells were arranged in columns with a small, round to slightly oval, eccentric nucleus approximately 4 μm in diameter. Medullary cells were polygonal in shape with a large, spherical, centrally placed nucleus approximately 5 μm in diameter.