A teenager who died after he is thought to have taken laughing gas at a party has been named in reports as Ally Calvert, 18.
The youngster was taken to hospital after he collapsed in the street in Bexley, south-east London at around 11.18pm on Saturday but died two hours later.
Police said he is believed to have attended a party where he may have been drinking alcohol as well as ingesting legal high nitrous oxide.
He was in cardiac arrest when officers arrived at the scene in Wolvercote Road.
The Metropolitan Police said: "Next of kin have been notified. A post-mortem will be scheduled in due course.
"At this stage the death is being treated as unexplained pending the findings of the post-mortem."
Reports said the victim was an electrician, from Sidcup, and was described by friends as "the nicest guy you will ever meet".
Research published earlier this year showed there had been a rise in the recreational use of nitrous oxide, known by various names including "hippie crack".
It is not illegal to be in possession of the substance although it is prohibited from being sold in England and Wales to under-18s if there is a risk they will inhale it.
The gas can make people feel relaxed, euphoric and giggly, leading to the nickname laughing gas. It is usually inhaled through balloons.
Abusing nitrous oxide can lead to oxygen deprivation resulting in loss of blood pressure, fainting and even heart attacks.
The research by respiratory consultant and neonatal paediatrician Dr Paul Seddon found that 7.6% of 16-to-24-year-olds in England and Wales have admitted to having tried balloons, which were described as "widely available" in shops that sell legal highs.
Seventeen people in the UK died between 2006 and 2012 after taking the legal high.