As Fernando Valenzuela of the Los Angeles Dodgers pitched his record-tying seventh shutout last week against the Cardinals, he looked as unassuming, as pudgy -- and as effective -- as when we last left him.

"I have not changed this year. I am the same person as ever," Valenzuela, age 20, from Etchohuaquila, Sonora, Mexico, said through an interpreter after his 5-0 victory over St. Louis that tied the National League record for shutouts in a rookie season. The major-league record of eight was set by Ewell Russell of the 1913 White Sox.

He has already won the hearts of fans. The phenomenon is called Fernandomania and it has dwarfed all else in this strike-torn season.

Said Tony DeMarco, Valenzuela's agent, "He is happy because it is happening when it is happening. But he is happy when it is not happening because it is not happening. It is exciting."

In the prestrike season, Valenzuela had an 8-0 start, five of those shutouts, with a 0.50 ERA. The media explored his background and the record books.

"Back then," Danny Ozark, the Dodger third base coach, said, "His popularity around the league was like the measles in grammar school. Everybody got Fernando Valenzuela."

The headlines around the country were almost as big as the subject's waistline. It seemed like opposing hitters needed a translator in the batter's box. They just didn't seem to understand his pitches.

But in his next eight starts, Valenzuela, a left-hander, had trouble. He was 1-4 with a 6.51 ERA and three no-decisions. It was a stretch that extended from six starts before the strike to two starts beyond it. He was having problems throwing strikes with his five-pitch repertoire, especially with his favorite, the screwball.

"It was his consistency and his control," said Ron Perranoski, the Dodger pitching coach. "Other teams were getting hits on his mistakes. It wasn't his stuff. The strike affected his control, too."

Now Fernando Valenzuela is 12-4, his ERA is about 2.40 and he has 156 strikeouts in 160 innings. He leads the majors in strikeouts and shutouts and is tied for most wins.

"I am on top of my game," he says.

"He's a damned phenom," said Whitey Herzog, the general manager/manager of the Cardinals. "We had four runs off of him in three games but we could have had one."

Said Keith Hernandez, the St. Louis first baseman and former NL batting champion, "This year, he's the best."

"I think he's a 3,000-year-old Inca chief who has come back," said Dodger Manager Tom Lasorda, digging into a bowl of postgame chili. "Who said Fernandomania is dead?"

In August, Valenzuela became the second rookie to start in an All-Star Game. He followed Detroit's Mark Fidrych (1976). In June, he became the first rookie to show President Reagan how to throw a screwball when he was invited to the White House.

And, of course, there was the honor of being on Playgirl magazine's list of 10 most desirable men. "I have no words to explain my feelings on that," said Valenzuela with a shrug and grin, and without the knowledge that Yankee owner George Steinbrenner finished two places in front of him.

"It surprised me at 20 years old to be doing all this," said Valenzuela. "I'm really surprised to be so famous in my first year . . . It is very difficult for me to say if it (Fernandomania) is still alive. The people are behind me 100 percent. You would have to ask the crowd. I'm winning and I'll keep trying."

DeMarco has been busy. The agent says Valenzuela has earned about $250,000 in endorsements this year and that he now charges $10,000 for an appearance. "That's just so he doesn't work at some grocery store openings or something," said DeMarco.

"We have probably turned down several hundred offers for endorsements this year. We do not want Fernando to be overexposed. Right now, he is doing a fruit juice commercial in the United States and he is doing commercials for a bank and a battery in Mexico. We're talking money with 7-Up right now, too."

DeMarco says his client, who is earning $42,500 this year, $10,000 over the major-league minimum, intends to negotiate with the Dodgers for a one-year contract after this season. DeMarco knows that his client's value may have no limit and doesn't want to be locked into a long-term contract. He also knows that, after 1982, he could use the weapon of salary arbitration.

"We are honest people," he said. "I hope the Dodgers pay Mr. Valenzuela what he is worth and with a smile."

Steve Brener, the Dodgers' director of public relations, is the man who was responsible for arranging all of those press conferences for Valenzuela earlier this year. He knows about the media and the mania.

"It was a lot like what Kansas City went through last year with George Brett hitting .400. I had never seen anything like it. I would say that our media requests are about half of what they were at the highest point. But Fernando is still going strong and I think it will grow to be like before, maybe even more," said Brener.

"I don't see Fernandomania ever ending. Everybody kept going out to see Koufax pitch, right?"

"I don't know if I'm pitching as well as I was at the beginning," Valenzuela said, looking out through eyes that are always busily observing the frenzy that surrounds him. "I haven't changed my style. Yes, the strike affected me, but I'm coming back. It was just a matter of time."