In this study, embryonic development of bronchus-associated lymphoid tissue (BALT) of chicken embryos as well as the effect of hormonal thymectomy induced by hydrocortisone acetate administration at post hatching period on the BALT were investigated light microscopically. For these purposes, a total of 80 chicken embryos from Avian Bred of different ages, and 200. 7-day-old chickens of the same strain were used. The embryos were used only to investigate the embryonic development of BALT. while the 7-day-old chickens, these were divided into two equal groups as norwreated. controls and hormonally thymectomized ex peri mentals, were used tor investigating the ettects of early hormonal thymectomy on the BALT, depending on the (actor "life ttme". In the lung sections ot the embryos aged between 6 to 18 days, any histological evidence of BALT development has not been observed At the post-hacthing period, between 4lh and days following HCA treatment, a definite reduction in the size of lymphoid follicles ot BALT İn companson with the non-treated controls was observed. Bronchus associated lymphoid units ot HCA-treated animals were smaller and lesser in number between 3ftl and 15th weeks after HCA-treatment Furthermore lymphoid tissues of these animals were observed as small lymphocytic eel) accumulation areas with poor cellulanty Germinal center (GC) formation has also generally been depressed and the poorly developed GCs were smaller and the units were with low cellularity. However, between 17'" and 25'n weeks following the HCA treatment, the BALT had quite similar appearance with those bl the controls. This morphological finding might be an indication that there is a reversal of the drawbacks ol early hormonal thymectomy with time, on the histological structure and cellular composition of the BALT in chicken.