OBJECTIVE: To study the effect of thyroid hormones on expression of GAP 43 in cerebellum of neonatal rats. METHODS Immunocytochemical method and image analysis were used to observe changes of GAP 43 in cerebellum of both normal and hypothyroid neonatal rats. RESULTS: The body weight (14.00 ± 3.03 g, 29.88 ± 8.92 g), brain weight (0.825 ± 0.078) g, (1.150 ± 0.110) g and thyroid hormones levels T 3: (0.26 ± 0.28) nmol/L, T 4: (10.00 ± 4.80) nmol/L of 15 or 30 day hypothyroid rats were significantly weaker than that of age matched euthyroid rats (33.00 ± 4.12) g, (80.00 ± 14.28) g; (1.098 ± 0.040) g, (1.430 ± 0.114) g; T 3: (1.052 ± 0.143) nmol/L, T 4:( 93.85 ± 7.92) nmol/L (P<0.001). The distribution tendency of GAP 43 in cerebellun was similarly in experimental pups as compared to age matched controls. In hypothyroid pups, the distribution tendency of GAP 43 in cerebellum was similar to that of age matched controls. The analysis of image shows that the grey values of GAP 43 was higher in hypothyroidism than that of normal pups at postnatal 15th days (171.35 ± 0.69 vs 160.83 ± 4.69, P< 0.001 ), but lower at pastnatal 30th days (149.60 ± 3.94 vs 159.42 ± 6.66, P< 0.01). CONCLUSIONS: Thyroid hormones affect expression of GAP 43 directly or indirectly. By regulating GAP 43, thyroid hormones have an effect on G 0 protein signal transduction system and may lead to brain developmental retardation.