TEHRAN, Iran >> "Happy nuclear deal," said Cyrus Razzaghi, a business consultant here, when he picked up the phone on Wednesday.

Razzaghi has been trying to make a business of organizing seminars and meetings for European and U.S. companies interested in Iran, but it had been a bit slow until now. Even though, technically, many of them could have done business here despite the sanctions, they feared damage to their reputations if they were seen working with the Islamic Republic.

He is expecting a surge in visits, but nothing concrete for a while at least. "Now, they are gathering information, doing due diligence on potential partners," Razzaghi said. "But this is not an overnight change."

He also warned that after 37 years of isolation, Iranian business culture was its own beast. "Sometimes Iranian businessmen ask for hundreds of millions of dollars in foreign investments, but refuse to pay for a feasibility study that costs less than $100. Many think only about now and not the future."

Over the last 20 months of nuclear negotiations, Iran’s leaders have been selling the country to their counterparts as a unique investment opportunity, flirting with giving Western companies access to one of the largest untapped markets in the world.

Now, a day after Iran, the United States and other world powers reached a historic agreement that could see the dismantling within months one of the strictest sanctions regimes in the world, it is clear that the opening, to the extent that there is one, will take some time to show results.

The most significant change, Razzaghi and many others say, will be when Iran regains access to the SWIFT system, the global electronic banking system that is essential for doing business in the contemporary world. "That means that all will be able to send money to and from Iran, and that is crucial for business," he said.

Potentially, the nuclear deal signed Tuesday will open up the world’s second-largest natural gas reserves and fourth-largest oil reserves, along with a decrepit and outdated energy sector that needs hundreds of billions of dollars and foreign technology and expertise to reverse a long-term decline. Iran’s airliners are decades old, in many cases.

The deal could also bring an underserved population of 76 million to multinationals and smaller companies. People are clamoring for iPhones, for example. But few here expect the gates to be flung wide open.

Some years ago, an enterprising entrepreneur in Tehran somehow received permission to open a McDonald’s, said Jamshid Edalatian, a German-educated economist. "Within days it was stormed by anti-American protesters," he said. "Those people are still around. Don’t expect to see the Golden Arches and Starbucks logos here in Iran anytime soon."

© 2015 The New York Times Company