In the den, on a bookshelf, an editorial cartoon sits enclosed in a simple black frame. The cartoon depicts a shifty-eyed Arab, labeled "Libya," clutching a briefcase full of cash and conspiring with his aides.

"Find out," the Arab says, "if Ronald Reagan has a brother who needs $220,000."

Ronald Reagan has a brother. But Neil Reagan, 71, says he doesn't need the money. As a matter of fact, he'd be just as happy if his 69-year-old younger brother -- whom he sometimes calls "Junior" -- weren't the man now most likely to be president.

Neil Reagan is a most reluctant candidate to replace Libyan lobbyist Billy Carter as the nation's "first brother."

"I don't know what I'd do with more money, really," he said, when asked if he would be tempted, like Billy Carter, to exploit his brother's political success for his own profit. "I don't owe anybody anything . . . There's nothing that somebody could offer me, even if it was the cleanest kind of brown-envelope operation, to get an appointment to talk to my brother . . .

"I wouldn't do anything to embarrass him. We've never done anything to embarrass each other," he said.

"I can't think of anything less exciting than having a brother as president of the United States," Reagan said the other day after playing 18 holes of golf on the Whispering Palms Country Club in Rancho Santa Fe, Calif., which doubles as his back yard.

"He was governor [of California] for eight years, and I was in Sacramento exactly five times. I only stayed there overnight once, and that was when he was inaugurated the first time. I wouldn't have stayed over that time, but the danged fool got himself sworn in at midnight. I would be very surprised if I would get to Washington five times."

Neil Reagan -- who resenbles his brother physically except that he is gray at the temples, thick at the neck and wears glasses -- is no black sheep.

He is not a drinker, gambler and all-around ne'er-do-well like Lyndon Johnson's brother, Sam Houston Johnson. He is not a failed businessman like F. Donald Nixon, who embarrassed his brother Richard by borrowing $205,000 from billionaire Howard Hughes. Nor is he like Edward and Robert Kennedy, presidential siblings with political ambitions of their own.

Neil Reagan is a retired advertising executive, with the emphasis on "retired." He gets furious if the phone rings after 5 p.m. He has been that way as long as anyone can remember. He spent 30 prosperous years working for the McCann-Brickson agency in Los Angeles, investing his money with wisdom and luck.

His own success has brought him the life he always dreamed of. He and his wife (who are childless) have a two-bedroom condominium in a country-club setting near San Diego. They spend much of the winter in an elegant old hotel in Baja, California much of the summer on a friend's schooner sailing north from Seattle.

He isn't going to let something as trivial as his kid brother's ambitions mess it all up.

"I have no intention of becoming a national figure out of this," he said. "At the moment, it's 'Who the hell is Neil Reagan?' I'd like to keep it that way . . . I hope to be among the lowest-profile first brothers of all time.

"I get real bothered about all this folderol. The family tells me, "There's a political occasion coming up, and you're going to be there, aren't you? I say, 'Hell, no!' So my brother's going to be there. I've seen him before. He's seen me before. Big deal. But if I don't go, then people may start saying, "Why isn't his brother here? Don't they like each other?' So I do go once in a while."

Make no mistake, Neil Reagan likes his brother and vice versa. Although they are not intimate politically, they are close personally. Before Neil moved away from Los Angeles, they would see each other almost weekly. The last few years, they have been sure of each other's company only at Thansgiving and Christmas.

Neil doesn't resent his brother's success, although he wonders why a man would want to run a country instead of a ranch. The brothers have a mutual respect bred of mutual achievement.

"The only way I'd have a role in the administration is if he'd ask me," Neil said, "and he'd never ask me. We've always had this thing about giving each other advice. Let's say I give him advice, and he follows it. And two months later I remind him that he did what I told him to do, he'll give me a quizzical look and say, 'You've got to be kidding. You never mentioned that to me at all.' It's just a game we play."

The two of them play so many games with each other that acquaintances often think they don't get along. When they are together in family settings, Neil said, they try to "cut each other's throat."

In fact, however, at key junctures in each brother's professional life, the other was there to lend a hand or give a push. Without Neil, Ronald Reagan might be a retired actor. Without Ronald, Neil Reagan might still be back home in Dixon, Ill.

As teen-agers in Dixon, Ronald and Neil (nicknamed "Moon" after he shaved his head in emulation of Moon Mullins, the comic strip character) were not the best of friends.

When the family moved from the south side of Dixon to the north side, Neil stayed in school on the south side, hanging out with "a bunch of guys staying out of prison by the skin of their teeth." Ronald attended the north-side schools, where the kids, according to Neil, were "a bunch of sissies."

But Ronald didn't forget his brother when he went off to Eureka College, 60 miles away, on a partial football scholarship. He helped Neil get a scholarship, too.

"We were in the same fraternity at college," Neil recalled, noting that in college he, in effect, assumed the role of younger brother. "He was the president, and I was a pledge. As pledges, we got paddled a lot. He seemed to feel it was necessary to hit me three times as hard as anyone else to show he wasn't playing favorites."

A few years later, Ronald, who had already started his own radio career, helped Neil get into radio. Neil used that opportunity as a vehicle to get to California and break into advertising.

A quarter-century passed before Neil could return the favor. In 1962, Neil browbeat Ronald, who was unemployed, into taking a lucrative position as host on a syndicated TV Western series called "Death Valley Days." The series was sponsored by one of Neil's clients -- Boraxo soap.

In testing his brother's ability to sell Boraxo soap, Neil made a significant discovery.

"We brought women in off the street and let them see Ronald's commercial among several others," he said. "They all agreed he was the best. They preferred him by a wide margin. They said he was honest. Why, they said they'd even vote for him if he ran for public office. And we hadn't asked them that."

In 1964, Neil, who had become a conservative Republican long before his brother, arranged for Ronald to give some speeches in the Los Angeles area for Republican presidential candidate Barry Goldwater. In turn, Ronald helped Neil get a job as Goldwater's top media adviser and producer of all the campaign's television commercials. And Neil helped get his brother's Goldwater speech onto network television -- and prevented Goldwater from vetoing the broadcast at the last moment.

It was that speech that launched RonaldReagan's political career.

A year later, Neil acted as an intermediary between the reluctant Reagan and the businessmen who wanted him to run for governor of California. Once the gubernatorial campaign began, however, Neil faded out. He is definitely not among the candidate's inner circle of advisers.

"I was a delegate at the convention in Detroit," he said. "No, I didn't want to be one. I'd been to conventions before. All that happens there is you get a hoarse voice, your feet swell and you don't get any sleep. The governor didn't ask me if I wanted to go. He announced it to me one day on the phone -- that he'd put my name on the list. I asked him where he got the muscle."

He would have preferred to stay far from the action, which is where he intends to spend most of the Reagan years -- if they are to be.

"I don't understand Billy [Carter]," he said. "Why would one member of a family constantly try to pull the rug out from under the other?

"Look, I could sit here and tell you stories about private conversations that involve things that could do great damage to Ronald, things that happened in his hotel suite during the convention. But I won't . . .

"I can't understand Billy."