Abstract: Objective To study the expression of GAP-43 hippocampal formation during brain developmental critical period of both hypothyroid and excessive iodine rats.Methods The model of hypothyroid and excessive iodine rats were established by pre-treatment with Thiamazole and Injection of iodine respectively,analysis of thyroid hormones,immunocytochemical method combining with image analysis were used to observe changes of GAP-43 in hippocampal formation of both normal and hypothroid as well as excessive iodine neonatal rats.Results The distribution tendency of GAP-43 in hippocampal formation was similarity in experimental pups as compared to age-matched controls.The grey values of GAP-43 were decreased in hypothyroidism(P<0.01),there were no statistical difference between excessive iodine pups and control(P> 0.05)of CA1 hippocampal formation during critical period of brain development.Conclusion (1)Excessive iodine could lead to thyroid hormones imbalance;(2)GAP-43 may be a molecular that thyroid hormones regulated directly or indirectly.By regulating GAP-43,thyroid hormones had an effect on Go protein signal transduction system,as occured in the wake of the second signal process and may affect the abilities of learning and memory.