Abstract: ObjectiveThis was the morphological report that leading to neuron developmental retardation in experimental hypothyroid rats.MethodThe effects of thyriod hormones deficiency on neuronal ultrastructure was observed during brain developmental critical periods of experimental hypothyroid rats by means of pre-treatment with Thiamazole.ResultsThe results showed that there were a large number of dark cells;The Mitochondria and Rough endoplasmic reticulm swelled, ribosome detached and isolated;parts of nucleus necrosis;The membranes of pre-synaptic and post-synaptic were not clear and synaptic cleft disappeared.ConclusionNeuronal ultrastructure was obviously abnormity in experimental rats.