Argentina’s security ministry says it has foiled a plot to use the Russian embassy’s diplomatic courier service to smuggle cocaine to Europe.
They then mounted a sting operation, replacing the drugs with flour and adding tracking devices.
The operation has led to arrests in Argentina and Russia, officials say.
Argentine Security Minister Patricia Bullrich said the cocaine found in the Buenos Aires embassy was worth 50 million euros ($62m; £45m) and was of very high purity.
She said it was destined for Russia and probably also Germany.
Ms Bullrich added that the alleged mastermind of the smuggling operation was in Germany and she expected this person to be arrested by German police.
Two people have so far been arrested in Argentina and three in Russia.
Russian security services had helped in the investigation and one of those arrested in Argentina was a serving police officer in Buenos Aires, she said.