Objective To observed the effect of pregnancy hyperthyroidism on spatial learning and memory in the rat offspring. Methods Female SD rats were randomly divided into normal control group (N group),mild hyperthyroidism group (A group),severe hyperthyroidism group (B group).The rat's hyperthyroidism model was established with levothyroxine sodium,then female rats and male rats were in one cage by 2∶1 ratio.After conception,levothyroxine sodium was continued giving to A and B group,N group received the same amount of saline.Thyroid function of rats offspring and Morris water maze test about their learning and memory ability were tested at 21^st day and 60^th day.The expression of neuronal nitric oxide synthase (nNos) was observed by western blotting technology,and the level of nitric oxide NO in the brain was tested. Results 1) The nNOS and NO of A and B groups were increased than N group,nNOS and NO of B group was increased than A group (P<0.05);2) The escape latency of group A was obviously prolong than that of group N in 21st day (P<0.05),but no difference between the two groups in 60th day (P>0.05).The escape latency of group B was obviously prolong in each phases than that in group A and N (P<0.05). Conclusion Pregnancy hyperthyroidism can increase neuronal nitric oxide synthase expression and the level of NO in the hippocampus of rat offspring, and cause rats offing cognitive dysfunction.