An anaesthetic vaporiser of the by-pass type has a sump 7 with an outlet flow path comprising a first run 27 leading upwardly, a second run 30 leading downwardly and a third run 32 leading upwardly again. A duct 23 connects the lower end of the run 30 with the lower part of the sump and in normal operation is sealed by the sump liquid. However, if the vaporiser is displaced such that the liquid flows from the sump into runs 27 or 30 of the outlet flow path the liquidinevitably drains from the duct 23. A flow path for inflowing gas is thereby opened directly to the downstream end of the run 30, preventing the over-administration of vapour from the device and removing any danger of liquid anaesthetic being blown through the device from the runs 27 and 30.
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