Radioactive material that was being kept at the Scottish nuclear power site Dounreay has been flown to the US.
Saturday's flight was the first movement of material held at the Caithness plant to the US since an announcement in February.
David Cameron, who was prime minister at the time, said the UK and US governments had agreed to an exchange of nuclear materials.
He said the UK would receive a type of uranium used to diagnose cancer.
But Highlands and Islands SNP MSP Maree Todd has criticised the weekend's flight and the level of secrecy surrounding the handling of nuclear material at Dounreay.
She said there should not be a need for an exchange of nuclear material, and the UK should be able to purchase what it requires for medical diagnoses.
Dounreay, near Thurso, is being decommissioned and the site cleaned up.
Most of the radioactive materials held there, such as fuel, are being moved to other locations, including Sellafield in Cumbria where it will be reprocessed or stored.
These shipments are being made by rail.
Other material has been returned to nuclear sites overseas.
During the 1990s, nuclear material was sent from abroad to Dounreay for reprocessing.
The customers included power plants and research centres in Australia, Germany and Belgium.
Under the UK-US deal signed earlier this year, highly-enriched uranium (HEU) stored at Dounreay is being sent to the US.
Saturday's flight took place under tight security from Wick John O'Groats Airport, which is about 30 miles (48km) from the Dounreay nuclear site.
It involved a US military Boeing C-17 transport aircraft.
Radioactive material from the civil nuclear industry has been transported by plane from the UK to other countries in the past.
The Nuclear Decommissioning Authority (NDA) provided Highlands and Islands Airports Limited with funding to upgrade Wick John O'Groats Airport in preparation for the US flights.
A spokesman for Dounreay said: "All nuclear materials are being removed from the site. This programme started in 2001 and we expect it to continue for a number of years.
"Our priority at all times is to comply with regulations governing the safe and secure transportation of nuclear material, both in storage and transit.
"Compliance with these regulations includes protecting information about routes, dates, timings and locations of nuclear material in any current or future transport."
MSP Ms Todd told BBC Radio Scotland: "I am disappointed we are hearing reports from people living up in Wick that this has happened, but got no official notification.
"I understand that there has to be some secrecy surrounding this industry, but I feel there is more secrecy than is absolutely required."
She added: "I understand that this material is weapons-grade in exchange for some medical radionuclides.
"I fail to understand why we cannot pay money for these things than have to supply them with weapons-grade nuclear material from the UK."