Every evening at closing time in Tucson's huge, swanky El Con Shopping Center, excited groups of Mexican families gather near the front entrance to compare notes on a day of furious shopping.

The Estaban Hernandez family -- father, mother, three sons, two daughters and Hernandez's mother -- were showing some of their purchases to friends from their home in Hermosillo, Mexico.

The list included a set of cutlery, eight pairs of straight-legged jeans, three blouses, four dresses, a computerized chess set, eight LP albums, 18 cosmetic items, a miniaturized TV set, five pairs of shoes and two auto stereo sets with speakers. They had spent a little more than $1,000 in six hours, Hernandez said.

Hernandez, an attorney, is among the thousands of Mexicans each week who come hundreds of miles north, by car, bus or train, from the border states of Sonora and Chihuahua to southern Arizona for a brief stay, as Hernandez said, "in the great American supermarket."

A recent study by the Arizona office of tourism said that Mexicans spent more than $408.4 million in Arizona last year, up from $347 million in 1978. Many southern Arizona businesses are routinely accepting pesos, hiring Spanish-speaking clerks and, in some cases, opening their doors on Sundays to lure weekend shoppers.

"It is pumping new blood into what otherwise might have been a failing local economy," said Tucson businessman Jack Hill. "We have stores here advertising weekend specials in Mexican cities hundreds of miles away."

"Without those sales to people from Mexico, we'd be dead," said Charlie Fowler, head of the Nogales, Ariz., Chamber of Commerce. "Eighty to 90 percent of all retail sales in Santa Cruz County [Nogales is the county seat] go south of the border." Spending in the county, just south of Tucson, grew from $154.6 million in 1978 to $176 million last year.

Fowler said that many customers take one of the three daily trains to neighboring Nogales, in Sonora, from as far away as Guadalajara, more than 1,000 miles south. Others come by bus, pick up their visitors' permit at the U.S. side of the border, and walk across.

They take back, he said, "everything from freezers to candy bars. It is impossible to pinpoint any one item. But in Mexico, that 'made in the U.S.A.' label still has prestige." Technically, he said, the shopping bags and carloads of U.S. merchandise entering Mexico are illegal but greasing the palms of Mexican customs officials smooths the way.

"I would guess that the 'mordida' [bribe] is not much more than $10 or $15 on a $300 TV," said Marvin Rivenburgh, Chamber of Commerce manager in Douglas, Ariz., a border town of 15,000, some 120 miles southeast of Tucson. Rivenburgh said that he'd never understood Mexican customs laws but he thought most purchases were legal.

Many stores in Douglas, which had $41.8 million in sales to Mexicans last year, clearly specialize in selling to the Mexican consumer.

One used appliance store near the wire fence separating Douglas from the Mexican town of Agua Prieta, population 50,000, sells ancient typewriters not even in working order for as much as $75 each.

"Schools and government offices buy them," said the store owner, who asked not to be identified. "I do not call it a ripoff because people are happy to buy them."

In the Douglas Shopping Center, Mexican-built Volkwagen Caribes (Rabbits), plus Fords and Chevrolets with the green license plates of several Mexican border states, sits in the parking lot outside the Safeway supermarket. When the Safeway was opened, said Rivenburgh, "it was the largest one in Arizona. I guess someone recognized the trade potential."

Store manager Joe Daneker said: "Forty percent of our business is with Mexican customers. We sell a lot of meat. It's becoming part of the diet there as more people get affluent. But most of our shoppers buy the whole range of items. They are about as selective as the middle-class American buyer."

Signs in Spanish help Mexican customers pick bargain items, Daneker said, and his store, like many in Arizona, places advertisements on Mexican radio and television and takes out ads in Mexican newspapers.

"One reason for the increase in spending," said Rivenburgh, "is the rate of inflation down there. It must be close to 30 or 35 percent. Our prices now seem reasonable to Mexican buyers and our quality is much better."

About the only item Mexican visitors do not take back, he said, is a luxury car. "They can buy a Ford or Chevy," he said, "but they can't take a Caddy back no matter how big the bribe. But I know a lot of Mexicans who'd like to buy one.

Both Rivenburgh and Fowler also point to the opening of a new copper mine at Nacozari, Sonora, 80 miles south of Douglas, as one reason for increased buying power. "But then, fishing and agriculture is booming in northern Mexico," said Fowler. "The labor market is strong, and we get the feeling sometimes that there are some petrodollars there, too."

While spending in towns such as Douglas and Nogales has become essential to the local economy, in larger and more distant Tucson it still has only a limited impact ($59.5 million last year). "But we get the more affluent people," said Bob Fisher, manager of Tucson's Lazy 8 Motel. "We get dozens of Mexican bankers, lawyers, doctors and businessmen every week. Our only trouble with them is the fact that they like to spend $100 bills and we have to keep a lot of cash on hand."