In the 1990s, the Iraqi government classified an army post housing the Balad Chemical Defense Battalion as a "sensitive" site, a designation that required advance notice and other formalities from U.N. arms inspectors if they wanted to pay a visit, effectively preventing them from conducting a surprise search.

This morning, however, a group of inspectors drove up to the site north of Baghdad armed with a new U.N. Security Council resolution requiring Iraqi officials to allow international arms experts to visit any facility they want.

As the U.N. experts arrived in a cloud of dust on the third day of renewed inspections, there was some shouting and scurrying among the guards, who did not appear to be expecting a visit. But in a key test of Iraq's compliance with the new resolution, gates that once would have remained closed were pulled open within minutes.

A spokesman for the U.N. Monitoring, Verification and Inspection Commission, which is conducting the inspections in conjunction with the International Atomic Energy Agency (IAEA), said the experts were interested in the base because the Iraqi military has said it trains troops there to defend against chemical, biological and nuclear attacks.

The spokesman, Hiro Ueki, said the inspectors sought to "determine what were the activities of the site since 1998," when U.N. arms experts last were in Iraq. Ueki said the inspectors wanted "to see if any evidence of chemical weapons agents or biological weapons agents were present and to see if there was any new equipment."

The inspectors spent about five hours at the site, ordering what appeared to be large ordnance crates to be pried open and requesting to look inside storage sheds. They also toted small, hand-held detectors, although it was not clear what the devices were testing.

The base, located about 55 miles northwest of Baghdad, is home to a battalion that reports to the military's Chemical Corps Directorate.

Karim Mohsen Alwan, a senior Iraqi military official who accompanied the U.N. team, said the inspectors "found nothing." Ueki would not comment on the inspectors' findings at the site.

The inspectors have said they will not publicly discuss their conclusions before reporting to their superiors in New York and Vienna, who in turn must report to the Security Council. The U.N. commission and the IAEA are required to provide an update of their work to the Security Council in late January.

In the 1990s, Iraq declared some facilities to be "sensitive" sites, requiring the United Nations to provide the government with advance notice of inspections and to limit the number of inspectors who could enter. The new resolution, passed unanimously this month, demands that inspectors be given "immediate, unimpeded, unconditional and unrestricted access" to all sites.

Inspectors from the IAEA visited two industrial plants south of Baghdad, which have been used by the military. The first was the Umm al-Maarik Co., known in English as the Public Company of the Mother of All Battles, the Iraqi term for the 1991 Persian Gulf War. The company is run by the government's Military Industrialization Commission. Plant officials say it produces light machinery, including spare parts for vehicles.

The second was the al-Milad Co., a facility previously known as al-Furat, which played a key role in Iraq's nuclear weapons program and was intensively investigated by U.N. inspectors in the 1990s. Iraqi officials contend that the complex now is used only for civilian purposes.

Before the Gulf War, researchers at al-Furat worked on developing centrifuges that could be used in the production of weapons-grade uranium. Recent satellite images indicate that a building planned years ago to house the centrifuges was completed after the inspectors left in 1998.

The director of the Umm al-Maarik Co., Hussein Hamoudi, said he was informed by the government that the inspectors would be visiting about an hour before they arrived. Ueki said the IAEA inspection leader told Iraqi officials on Friday night that inspectors would drop by the site to remove video surveillance equipment installed in the 1990s.

U.N. inspectors first arrived in Iraq in 1991, shortly after the Gulf War ended. They destroyed tons of chemical and biological weapons and have been credited with dismantling the country's nuclear weapons program. But the monitoring ended in 1998 because of disputes over the inspectors' access to sites and Iraqi objections that the United States used some inspectors as spies.

Team leader Jacques Baute, center, along with two other U.N. nuclear experts, investigates the al-Milad Co.