Three UK ex-servicemen have been given compensation after they were given LSD without their consent in the 1950s.
A "volunteers programme" started at Porton Down in 1916
The men volunteered to be "guinea pigs" at the government research base Porton Down after being told scientists wanted to find a cure for the common cold.
But they were given the hallucinogen in mind control tests, and some volunteers had terrifying hallucinations.
The Foreign Office said the secret intelligence body MI6 had made the settlements after legal advice.
The out-of-court settlements are thought to be under £10,000 for each of the men.
In a statement issued later to the BBC News website, the Ministry of Defence said it did not make any admission of liability in respect of the settlements.
The statement added: "The Ministry of Defence is very grateful to all those whose participation in studies at Porton Down made possible the research to provide safe and effective protection for UK Armed Forces."
A spokesman for the Foreign Office, which oversees MI6, said: "The settlement offers were made to the government on behalf of the three claimants which, on legal advice, and in the particular circumstances of these cases, the government thinks it appropriate to accept."
The men had volunteered for experiments at the government's chemical warfare research base at Porton Down in Wiltshire in 1953 and 1954.
Following the settlement, Don Webb, who was a 19-year-old airman at the time, told BBC Radio 4's Today programme: "I think they grudgingly acknowledged that they did something wrong.
"They stick to the old maxim: never apologise, never explain. But I think in this case they have decided to pay some money. I think that is as near to an apology or an explanation I'll get."
Both he and fellow serviceman Logan Marr, a former shepherd from the Scottish highlands, suffered hallucinations after they were asked to drink a clear liquid.
The third man did not wish to be named.
The research was carried out after British and American governments thought the Soviet Union had developed a "truth drug" which could compel spies and servicemen to yield up important secrets.
MI6 scientists decided to test LSD, the closest thing they thought they had to a truth drug, on volunteers to see how they reacted.
Alan Care, a lawyer who represented the three men, said: "As far as we are aware, these are the first settlements by the secret intelligence services for a personal injury action."
He added that a request that documents relating to the case be put into the public domain had been refused.
Some volunteers at the base did not find out they had been given LSD until 50 years later. Thousands of servicemen and women have volunteered in the testing of defences against chemical and biological attacks at the Wiltshire military base.
Research began in 1916 using a "volunteers programme", and up to 20,000 people took part in various trials in the 50 years up to 1989.
Last October, the government was found guilty of breaching the human rights of former soldier Thomas Roche, who claimed he developed health problems as a result of mustard gas and nerve agent tests in 1962 and 1963.