: As part of our recent study on mitochondria injury and increased apoptosis in lymphocyte spleen cells of young chickens fed high fluoride (F) diets containing 400, 800, and 1200 mg F/kg, the spleen growth indexes in the two high F groups II and III were found to be markedly lower than in the control group. Histopathologically, in these two high F groups the number of splenic nodules and the lymphocyte population with splenic nodules and periarterial lymphatic sheaths in white pulp were obviously decreased along with the widened splenic sinus and erratically arranged lymphocytes in red pulp. Moreover, lymphocytes in G 0 /G 1 phase were significantly increased, whereas lymphocytes in S phase and G 2 +M phase and the proliferation index (PI) were markedly decreased in the high F groups II and III. The results confirmed that high dietary F in the range of 800~1200 mg/kg inhibited the proliferation of splenocytes and suppressed spleen development in young chickens.