Detectives investigating the sudden death of a 17-year-old boy say a lethal batch of drugs could be circulating in the Rhondda.
Officers were called to the boy's home in Treherbert on Monday. He died after being found in cardiac arrest.
A short time later, a second male from Treorchy was taken to hospital where he remains in a serious condition.
Police say two people have been arrested in connection with the death.
They believe both of the men had been supplied with drugs, possibly opiates, at the weekend that had caused them to fall ill.
They warned people in the area to be especially cautious.