ObjectiveThis study aims to investigate the effectiveness and potential mechanism of Erxian Decoction （二仙汤） for late-onset depression （LOD）. MethodsForty Wistar male rats of 7-8 weeks were divided into 20 each of normal group and youth depression group. Sixty Wistar male rats of 20 months were divided into 20 each of elder group， LOD group and Erxian Decoction group. The rats in youth depression group were modelled with chronic unpredictable mild stress （CUMS） for 6 weeks to build a depression model， and the elder rats in LOD group and Erxian Decoction group were modelled with CUMS for 6 weeks to build a LOD model， with no interventions in the normal group and the elder group. Gastric administration was carried out at the same time of modelling， rats in Erxian Decoction group were given 8 g/（kg·d） of Erxian Decoction by gavage， and rats in the normal group， youth depression group， elder group， and LOD group were given 4 ml/（kg·d） of pure water by gavage， for 6 weeks in each group. Sugar water preference experiment and forced swimming experiment were used to evaluate the depression-like behaviour of rats， absent field experiment was used to evaluate the spontaneous activity ability of rats， and T-maze experiment was used to evaluate the cognitive function of rats； Western blot assay was used to detect neuronal nuclei （NeuN）， nestin， doublecortin （DCX） in hippocampal tissue， and zonula occludens-l （ZO-1）， folate receptor α （FRα）， reduced folate carrier （RFC）， and protoncoupledfolate transporter （PCFT） protein expression in choroid plexus； immunofluorescence was used to detect the number of double-positive cells for Ki-67/nestin， the number of double-positive cells for 5-bromodeoxyuracil nucleoside （BrdU）/DCX in hippocampal dentate gyrus， and the protein expression of ZO-1， FRα， RFC， and PCFT in choroid plexus tissues； ELISA technique was used to detect plasma， cerebrospinal fluid， 5-methyltetrahydrofolate （5-MTHF） in hippocampal tissues； Pearson correlation analysis was used to correlate the 5-MTHF content in cerebrospinal fluid and hippocampal tissues and the expression of nestin， DCX， and NeuN proteins in hippocampal tissues of rats in youth depression group and LOD group. ResultsCompared with normal group， rats in youth depression group and LOD group showed reduced sugar water preference， reduced total distance travelled in the absent field， reduced number of times through the grid， prolonged forced swimming immobilisation， reduced hippocampal NeuN， nestin and DCX protein expression， reduced number of Ki-67/nestin double-positive cells in hippocampal dentate gyrus， reduced number of BrdU/DCX double-positive cells in hippocampal dentate gyrus， and reduced expression of ZO-1 and FRα proteins and fluorescence intensity （P＜0.05 or P＜0.01）； the correct rate of T-maze on days 3 and 4 in the rats of youth depression group and on days 2 to 4 in the rats of LOD group significantly decreased， and the content of 5-MTHF in the cerebrospinal fluid and hippocampal tissues of the rats of LOD group significantly decreased as well （P＜0.05 or P＜0.01）. Compared with youth depression group， rats in LOD group showed a decrease in total distance travelled in absenteeism， number of times through the grid， a significant decrease in the correct rate of the T-maze on day 1-4， a decrease in NeuN， nestin， DCX protein expression of hippocampal tissue and the number of Ki-67/nestin double-positive cells， BrdU/DCX double-positive cells of hippocampal dentate gyrus （P＜0.05 or P＜0.01）. Compared with LOD group， rats in Erxian Decoction group had elevated sugar-water preference and total distance travelled in absenteeism and number of times through the grid， shorter forced swimming immobility time， significantly higher correct rate of the T-maze on day 3-4， elevated expression of NeuN， nestin， and DCX proteins in hippocampal tissues， increased number of Ki-67/nestin double-positive cells and BrdU/DCX double-positive cells in hippocampal dentate gyrus， and increased 5-MTHF content in cerebrospinal fluid and hippocampal， and ZO-1， FRα， RFC， PCFT protein expression and fluorescence intensity increased in choroid plexus （P＜0.05 or P＜0.01）. Correlation analysis showed that there was a positive correlation between 5-MTHF content in cerebrospinal fluid （r = 0.466）， hippocampal tissue （r = 0.522） and nestin expression in hippocampal tissue （P＜0.05）. ConclusionErxian Decoction could improve depressive-like behaviour and cognitive impairment in LOD model rats， and its mechanism may promote hippocampal neurogenesis by alleviating structural damage to the choroid plexus of the brain tissue， and improving impaired choroid plexus folate transport.