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."

The skeleton was unearthed during a large-scale excavation of the Swinegate area of York in 1990.
The York Archaeological Trust (YAT) said more than 100 burial sites were discovered, many dating from between the 9th and 11th Century.
The skeleton and coffin are being displayed at the city's Jorvik Viking Centre.
York was occupied by the Vikings from the late 9th Century until the middle of the 10th Century, when it was absorbed into the Anglo-Saxon Kingdom of England.
The Swinegate dig explored the churchyard of the former church of St Benet, which had stood on the site from the 8th to the early 14th Century.
Unusually, a number of timber coffins had survived.
Sarah Maltby, director of attractions for the trust, said: "As we found in the Coppergate dig of the late 1970s, York's waterlogged soil conditions preserved the timber of several coffins, including this one, so what our archaeologists unearthed represents a series of previously undisturbed burials with complete skeletons, some of which date back more than 1,000 years."
She said the condition of the wood gave the coffin a national significance, as so few similar examples exist.
The coffin was made for a young woman, estimated at being between 26 and 35 at her death, and dates from the late Viking period.
The YAT said recent analysis of the bones revealed the woman had inadequate nutrition or disease as a child and a degenerative joint disease in the spine and hips.
There was no indication of what caused her death.
Radioactive material that was being kept at the Scottish nuclear power site Dounreay has been flown to the US.
[NEXT_CONCEPT]
The skeleton of a young Viking woman found in an intact timber coffin has gone on display for the first time.