Abstract： Objective To observe Semaphorins-3A and synaptophysin P38 expression in hippocampus of developing rat induced by status epilepticus.Methods 320 SD rats were divided into four groups (P7,P14,P21 and P28) according to day -old after birth (7d,14d,21d and 28d).Rats in each group were randomly divided into model group (SE group) and saline control group (NS group).SE was induced by Pentylenetetrazol (PTZ).Semaphorins-3A and synaptophysin P38 expression were determined by immunohistochemical staining on 1d,7d,14d,21d and 28d after SE in hippocampal CA1 of rats.Results Semaphorins-3A-positive cells could be seen in the hippocampal granule cell layer in all rats.Semaphorins-3A expression tended to decrease with the increasing of day-age,especially in P7 group(91 552.68 ± 4664.69 ).No matter how day-age,Semaphorins-3A expression was similar to that in NS group and was obvious reduced in 7d after SE(56 938.84 ± 5688.47 ).Meanwhile P38 expression in P7-day-age rats had had been gradually increasing between 1 day and 14d (5413.18 ±48.77,6223.40±29.19,6902.94 ±78.51 ) and then stabilized gradually on 21d(7523.42 ± 62.94) after rats were tested.P38 expression in other day-age rat was relatively stable on the same level in physiological state.On the other hand P38 expression in the hippocampal CA1 region of P7,P14,P21 and P28 rats was significantly higher than that in normal rats between 1day and 28day after SE episode(P＜ 0.05 ),and reached a peak on 14 day(8408.35 ± 55.73 ).Conclusion Semaphorins-3A and synaptophysin P38 involved in hippocampal synaptic plasticity of rat in developing stage and epilepsy.