1. Post-hatching developmental changes in the ultrastructure, cell area and histological RNA content of duodenal absorptive epithelial cells were observed in 1, 10 and 60-d-old White Leghorn (WL) and broiler (BR) chickens, with special reference to the mitochondria (Mt). 2. Epithelial cells in 1-d-old WL and BR had a well-developed Golgi area and a dense cluster of rod type Mt near the cell surface and at the infranuclear region. In BR, cells included well developed profiles of endoplasmic reticulum but fewer supranuclear vacuoles than those of WL, in which numerous free ribosomes were also found. Some Mt in BR showed a bud-like protrusion from the main body (tadpole type Mt). These suggest that the fine structural maturation of the epithelial cells in WL is involved in the process of cell maturation but epithelial cells of BR have almost matured at hatching. 3. In 10-d-old WL and BR, supranuclear vacuoles had disappeared and Mt increased in number. In WL, in addition to a decrease in free ribosomes, Mt developed to the tadpole type and further to dumbbell type ones. In BR, Mt also aggregated at the perinuclear region and some of them had developed from dumbbell shapes to a thick doughnut type. These findings indicate that epithelial cells in both breeds are more developed ultrastructurally than those in 1-d-old and that epithelial cells in BR are more activated for digestive and absorptive functions than those in WL. 4. In 60-d-old WL and BR, Mt showed various types with thinner matrices than 10-d-old, suggesting that the cell structure and function had reached a stable state. 5. BR revealed higher values for cell area and RNA content than WL at every age and RNA content of both breeds were maximal at 10 d, followed by 60 d. 6. Mt changed their shapes from rod type to tadpole type. The latter then developed to doughnut type via dumbbell type, related to an increase of epithelial cell functions. 7. Epithelial cells of BR are thus more highly activated than WL cells at each age and reach morphological and physiological maturation around 10 d.