In a telephone interview with Reuters on Monday, International Atomic Energey Agency (IAEA) chief Muhammad al-Baradai said: "We went there today and we took environmental samples. Tomorrow we will also visit places in the country to look at relevant equipment.
"The fact that we got prompt response and access is, I think, something positive."
Earlier this month, Reuters reported that two satellite photos from August 2003 and March 2004 showed Iran had removed all the buildings and much of the topsoil at Lavizan, which is located near a military installation.
The Bush administration, which has branded Iran part of a global "axis of evil" seeking illegal weapons, said this suggested that Iran was pursuing a secret nuclear-weapons programme.
Iran denies that Lavizan was a nuclear site and insists it has no weapons programme.
"The fact that we got prompt response and access is, I think, something positive"
Al-Baradai, in Moscow for talks with Russian officials, declined to give details on the inspectors' initial impression of the site, and said it would take some time for the results of the samples to come back from the laboratories.
The IAEA had taken environmental samples to test for traces of nuclear materials that might indicate signs of undeclared activities.
The agency found traces of enriched uranium at various sites in Iran last year and has yet to determine their origin.