A second set of tests is being carried out to assess the impact of toxic gas should it be released by terrorists on the London Underground.
St John's Wood station will remain open
For the second consecutive Sunday, harmless sulphur hexafluoride will be released inside the Jubilee Line's St John's Wood station in west London.
The station will remain open as scientists use specialist equipment to check gas levels in the building.
The results will be analysed by Ministry of Defence scientists.
Transport Secretary Douglas Alexander stressed the trial was not a reaction to any threat increase or a measure to enhance security at Tube stations.