Mr Maddison died at Porton Down in 1953
A second inquest into the death of a soldier during defence experiments at Porton Down more than 50 years ago has opened.
Ronald Maddison was 20 when he died after he was reportedly exposed to chemicals at the Ministry of Defence laboratories in Wiltshire in 1953.
Last year the High Court ruled a new inquest should be held.
Coroner for Swindon and Wiltshire David Masters said the re-examination of the death was a "unique occasion."
"No coroner or jury has been required before this day to investigate a death that took place so long ago as this one, " he told the jury.
"It will be necessary for us all to take a journey back to the early 1950s. Some of you may remember those years, some of you obviously will not."
Mr Masters said that on 6 May 1953 Leading Aircraftman Maddison was one of a team of six servicemen who entered a chamber at Porton Down.
LAC Maddison was the fourth to have a piece of uniform material loosely wrapped around his left arm and 20 drops of 10mg of Sarin applied to it with a pipette.
The servicemen were then left in the chamber for 30 minutes, observed and asked how they felt.
LAC Maddison went into the chamber wearing a respirator at 1017 BST and initially said he felt fine, but at 1040 BST he began to experience breathing difficulties and was sent out to a bench in the open air.
Minutes later, his condition worsened and he said he could not hear. Mr Masters said he was "gasping" for breath.
He was given an antidote injection and at 1047 BST was administered oxygen after being taken to the base medical centre.
At 1100 BST he was described as "ashen grey" and no pulse was felt. He was given further antidote injections into his veins and then intra-muscularly, and eventually an injection of adrenalin into his heart.
But at 1330 BST attempts to resuscitate him were abandoned.
He had become the first, and only, person to die in trials at Porton Down and the hearing was told all experiments then ceased, with Prime Minister Winston Churchill being informed of the decision.
But after an August 1953 report into the events the dosage used in Sarin trials was lowered to only five milligrammes put on to clothing and skin under the new guidelines and testing resumed.
The coroner said the fact that the inquest was being held with a jury would ensure "full transparency".
But he said that ongoing security issues, and the fact that certain papers relating to the issue were still classified 50 years on, meant sensitive sections of the inquest would be held behind closed doors.
The hearing, at Trowbridge Town Hall, is expected to last for eight weeks and will hear evidence from experts, servicemen and eyewitnesses.
Last year Lord Chief Justice Lord Woolf quashed the original inquest verdict of death by misadventure and said a new inquest was needed in the interests of justice.
Mr Maddison's sister and members of the Porton Down Veterans Support Group attended the hearing on Wednesday.