Another chapter was added to the storybook life of Los Angeles Dodger rookie pitching sensation Fernando Valenzuela yesterday when he attended a state luncheon at the White House honoring Mexican President Jose Lopex Portillo.

Poised and appearing more a banker than a pitcher in a charcoal-gray pin-striped suit, Valenzuela stood outside the diplomatic reception room and courted the press while John Gavin, U.S. ambassador to Mexico, acted as interpreter.

"Of course, I'm very proud, and I'm very honored, to be invited by hte president of the United States," Valenzuela said. "Of course, it's doubly important for me because we have my president here, also, and I'm looking forward to seeing both of them today."

Valenzuela passed through the South Portico, answering a battery of questions hurled athim in Spanish and English. He autographed several baseballs handed to him by his business manager. Antonio DeMarco, to present to the presidents and members of the Mexican press.

When Secretary of State Alexander M. Haig Jr. arrived, Haig heartily clapped his arm over the pitcher's shoulder.

"Are you a baseball fan?" someone asked Haig.

"You betcha!" he shot back.

The questioning turned to politics and Haig was asked if the United States would cut off military aid to Israel as a result of that country's bombing strike against an Iraqi nuclear reactor. The secretary dodged the question and the foursome quickly headed upstairs to meet with the presidents for a nearly three-hour luncheon.

Reagan, who once recreated Chicago Cub games on radio and played Hall of Fame pitcher Grover Cleveland yalexander in a motion picture, called Valenzuela a great pitcher.

Earlier this week, Valenzuela, 20, had pondered this upcoming honor and the whirlwind events that have changed his life so dramatically.

"It is like a fairy tale story, with the fact I am in the major leagues being first," he said through his interpreter Jaime Jarrin, the Spanish language voice of the Dodgers. "I never dreamed I'd be with the Dodgers.

"Since they signed me I'd been longing to come to the major leagues. The success I have had at the beginning is like a fairy tale," he said. "All the press and commotion is something out of this world. I am so deeply emotional, it is hard to explain."

His face is sometimes pensive, sometimes impish. His voice registers a range of emotions -- joy, awe, wonderment. He leans his broad, bullish upper torso forward, as if he's about to share a secret. Then he says he never dreamed he would reach the major leagues.

As a young boy growing up in the tiny Mexican village of Etchoahuaguila, his only dream was to play baseball in the Mexican League. He said God gave him the ability, the Dodgers gave him the opportunity.

"Now that I am here I am proud to be Mexican and to be playing in the major leagues . . . Regarding the game, the most difficult part was to reach the major leagues. Now that I'm here I will do my best to stay."

He had eight straight victories at the start of the season, including five shutouts. Recently there have been losses.

He said he is slowing down, finding it "impossible ot keep the same pace I had at the beginning." He said he doesn't fear his opponents are catching on to him, but "probably yes, if they face me a second time they know the way I pitch. But I know them also.

"I don't think I'm supposed to go out and win every single game."

He speaks wistfully about his slowly emerging status as a symbol of Mexican hope, a goodwill sports ambassador. He longs for the day when he can devote more time and attention to social causes in the Hispanic community, as such players as Roberto Clemente and Manny Sanguillen did before him.

Recently he met with a terminally ill Mexican teen-ager who had only three-weeks to live, he said. The youth's dying wish was to have his picture taken with Valenzuela. Plans are also under way for Valenzuela to do a series of public service radio spots encouraging youngsters to stay in school. He never attended high school.

He says he wants to set an example for youth "to be responsible in anything they do, especially the Mexican children of Southern California."

Among his personal goals is a desire to study English. "If I could understand English, I could enjoy more of the nice things they are saying about me," he said, laughing. Another dream is to build a house in Mexico for his family.

For himself, he says he has no specific wish. In Valenzuela's storybook life, all of his wishes have already come true.