Despite long lines at some branches this morning, Mexico's banks reopened without incident today for the first time since they were nationalized last week.

Business appeared normal, even lighter than usual in many locations as radio broadcasts urged customers to conduct only urgent transactions today.

There were fears that six weeks of increasing economic chaos would cause a dangerous run on the banks today, but by early afternoon newspaper headlines proclaimed, with at least temporary justification, that "The People Responded" to government pleas for calm.

At the airport, which has become a kind of barometer of the foreign exchange crisis, there was still considerable confusion and apprehension surrounding the changing of dollars.

The rate was fixed over the weekend at 70 pesos to the dollar after surging well over 100 at the end of August.

Travelers leaving the customs area passed booths set up by the Bank of Mexico with hand-lettered signs first telling them: "Dear passengers, Here you must exchange your dollars or travel checks USA to Mexican pesos." And then, "You need to change only as much as you will need."

Neither incoming nor outgoing passengers said they had been subjected to any special searches for currency, and Bank of Mexico employes said that reports that everyone coming into the country would have to change all his dollars into pesos were misleading.

"The 'must' on the sign comes from Saturday and Sunday when no other banks were open," said cashier Ruben Castro.

Another problem was soon evident, however, as departing tourists and Mexicans inquired about buying dollars. For the moment there is no way to purchase American currency legally at the airport, even to change back pesos bought on the way into the country.

One frustrated Mexican woman who is leaving for a vacation in the United States Tuesday was looking for some place to purchase the $250 she is allowed to take out of the country.

"The banks in the center of the city say to come here to the airport. The banks in the airport say go to the center," she said wearily as Castro gave her a helpless shrug. "You don't know," she said. "Nobody knows. That's all I wanted to know."

Raul Munoz Contreras, director of the Bank of Mexico operation at the airport, said that for the moment no dollars are being sold, and "beyond that we don't have instructions," but he supposed that in two or three days a system would be in effect allowing tourists to reconvert pesos to dollars on their way out of the country.