Nitrous oxide was a type of gas, known on Earth as laughing gas.
The First Doctor once encountered homicidal earthworms which excreted nitrous oxide, causing him to laugh against his will. (PROSE: A Big Hand for the Doctor)
The Third Doctor synthesised nitrous oxide and used it to drive out the Gaderene who were possessing the residents of Culverton. (PROSE: Last of the Gaderene)
On Mars, the Fifth Doctor and Peri Brown detected the smell of nitrous oxide in the air, which Peri described as being "like chemistry class." (AUDIO: Red Dawn)