Inspectors from the UN's nuclear agency have visited Iran's Arak heavy water production plant for the first time in more than two years.
The visit comes two weeks after a six-month interim agreement was reached between Iran and world powers over the country's nuclear programme.
Iran has agreed to curb some nuclear activities for six months in return for sanctions relief.
It has promised not to commission or fuel the Arak reactor during that time.
The Arak heavy water production plant is designed to supply a research reactor under construction nearby.
The Arak reactor is significant because if completed, it could open the way for the reprocessing of plutonium - a potential step towards a nuclear weapon.
Some world powers say Iran's uranium enrichment programme is geared towards making a weapon, but Tehran insists it only wants to be able to produce nuclear energy.
The inspection is the first real test of Iran's commitment to the interim agreement it signed with world powers two weeks ago, says the BBC's James Reynolds in neighbouring Turkey.
The inspectors are due to return to their headquarters in Austria after completing the one-day inspection.
Under the international deal, Iran will receive some $7bn (£4.3bn) in sanctions relief while talks continue to find a more permanent agreement.