Officers searched a flat in Lily Street, in the Dalmarnock area of Glasgow around 8am yesterday and found methylone powder, which can be used to make fake ecstasy tablets.
Amphetamine, cannabis and drug making equipment was also discovered, police said.
A 23-year-old man and a 21-year-old woman were arrested in connection with the find which is said to have a combined street value of £900,000.
A report will be sent to the procurator fiscal and the man and woman are due to appear at Glasgow Sheriff Court on Monday.
Detective Sergeant Joe O’Neill said: “This was a significant recovery and shows our commitment to keeping people safe.
“I would encourage anyone who has any information about the sale or supply of such drugs to contact police on the non emergency number 101, or contact Crimestoppers on 0800 555 111 where information can be given in confidence.”