The busloads were trucked in from Northern California and thousands more arrived by plane, car or camper from almost everywhere. They all came to gamble money at the world's largest casino, the MGM Grand Hotel-Reno. The casino that costs a modest $131 million for 26 stories, 1,015 rooms, 100,000 square feet of gambling space - that's more than twice the floor room at the Grand in Vegas. Accommodations are priced at $35 a day for Basic Gamblers and $250 a day for Basic Eastern Potentate. A local newspaper described the opening of the emporium as "The Second Coming." And when the security guards feared on Wednesday that the impatient crowds might break down the glass doors, the casino opened, 45 minutes ahead of its scheduled noon premiere.

The Grand did not fare so well on its opening day. The lights went out at the blackjack table, one guest had to pry open the door to his room with a knife, slot machines and security alarms malfunctioned, elevators got stuck between floors, and down-on-their-luck gamblers had to wait in line up to an hour for $2.75 cheeseburgers.

"If his had been a cruise ship," said one blackjack dealer, "it would have sunk in sight of the dock."

In Reno, the second coming got off to a slow start.

High roller Arnie (no real name, no last name, please) left his brokerage firm in San Francisco for three free days on MGM including a sitting room, wet bar, room service, complimentary wine. The hotel obtained Arnie's name from the list at another plush hotel where Arnie frequently stays on gambling jaunts.They checked his credit out and told him to come on up on them. Almost everyone here this week was an Arnie gamble-a-like or a media-type.

Arnie is a card counter at the blackjack tables. At least, he says, "I am when I'm sober." At MGM Grand, four decks are dealt out of a chute, called a "shoe," making it impossible for an experienced player to keep an accurate count on the cards played. His first night here he played at the MGM Grand and, drinking two shots of Jack Daniels neat during a three-hour card-playing period, wound up $700 ahead. The difficulty of getting a drink balanced out the loss of the single deck. He lost half of the $700 the next morning in three minutes. He lost the rest, plus $400 more, the same afternoon at a downtown casino where the drinks came as quickly as the cards. He lost $400 more later that night back at the MGM Grand.

"I like the atmosphere here," he says. "There are so many tables (102) here that the dealers don't rush you. It's a nice place to lose your money."

Arnie had planned to bring his wife with him to the hotel opening, he says, half-apologizing for his luxurious room.A daughter visited them unexpectedly, and the wife, who doesn't like gambling but apparently still likes Arnie after 20 years of marriage, decided to stay home.

Arnie says he gambles twice a month, always for moderately high stakes. He used to have a bookie in San Francisco, but dropped him after the bookie was raided. By "gambling" Arnie means sitting down and playing cards. He does not count his weekly trips to the San Francisco-area race-tracks as gambling. They're "recreation." He gives no figures about what he wins or loses, other than to say, "I do all right when I understand the situation."

Arnie thinks the MGM Grand opened "before they worked the bugs out" but says he'll come back as soon as they do. He likes the atmosphere and thinks his wife, whom he calls a "tennis freak," will enjoy the five tennis courts.

The new hotel complex is a kind of adult Disneyland where nongambling family members can watch jai-alai, play tennis, bowl or have their pictures taken with an African lion (Hotel officials estimate 30,000 persons passed through in the first 24 hours.

Ed and Vera Smith of Roseville, a distant suburb of Sacramento, took a one-day chartered bus ride to Reno to play slot machines and, in Ed's case, a little poker. The retired couple brought $135 with them to gamble, with $20 more for meal money.

Ed Smith drew out twice on good pots at the poker table and came away $100 ahead about the same time Vera was putting the last of her $55 share of the gambling money into the slot machines. Ed repaired to the blackjack tables and Vera to the Stage Door Buffeteria, an experience which did neither of them any good. After 45 minutes in the buffeteria line, the normally taciturn Vera was telling anyone who would listen that she would not come back to the MGM Grand but would go to Harold's Club or Harrah's instead. Both Smiths are used to the low-priced, round-the-clock food of the Reno downtown area two miles away, where casinos advertise 99-cent breakfasts and serve good food to get customers inside the clubs. But Ed, who by the time Vera had reached the buffeteria and its $1.75 egg salad sandwich, had lost $82 of his poker winnings at blackjack said that both of them would be back to the MGM Grand Hotel.

It was a difficult week for Arnie, Ed and Vera and a mixed experience for the City of Reno which did the sort of things to smooth MGM's coming that southern towns used to do to lure New England industries. The master plan was changed, the zoning was altered, city land which contained the animal shelter was sold to MGM at a fraction of its price.

That sort of thing.

MGM showed its appreciation with a lavish grand opening party that featured dancing to Harry James. The dress was "black tie opional" in a town where tuxedos are almost never worn.

It was no accident that Arnie left a happier sort than Ed and Vera Smith. MGM knew that the high rollers wouldn't return unless they felt comfortable the first time, and the movis company lavished its preopening intentions on the hotel.

"Anyone can see that the casino has just opened, but it seems like the hotel has been here for years," said Arnie. His only complaint about the hotel was that a maid knocked on the door while he was taking a shower to give him a "Do Not Disturb" sign.

Not much of a complaint from a man who dropped $800.