Next time you check into a hotel, you may get a room key that looks like a domino.

Many large hotels these days are installing sophisticated new lock systems that don't even use conventional keys. They use plastic inserts or no key at all -- guests simply punch out their special room code on a digital push-button lock.

Security is the main reason for the change. Lost room keys are a problem. For hotel guests, there's the inconvenience of having to obtain another key and the worry of possible theft. For the hotel, there's the expense of having to make duplicate keys and a continual security problem.

The new systems offer one great advantage over the conventional locks: Replacing a "key" or changing a lock is a simple operation, and that means that much tighter control is kept over who has access to a room.

Just being installed at the Sheraton Bal Harbour here is the VingCard system from Norway, also in use at the Peachtree Plaza in Atlanta. Instead of a conventional key, Sheraton guests are given a plastic card with pre-coded holes punched into one end. To open his door, the guest inserts the card into a slot above the door handle.

If a guest loses his card-key, or if one is stolen, both lock and key can be replaced in 15 seconds. The old card is instantly useless and the cost of replacement is minimal -- a set of five new cards and lock costs $1.20 plus the minimal installation labor time.

Cost of making a duplicate metal key, on the other hand, is about 70 cents and the old key might still be floating around -- perhaps in the hands of a thief.

Master card-keys used by chambermaids and other hotel personnel also can be replaced quickly under the VingCard system, as in most other systems.

"Before we installed this system, there was no way to control the number of 'keys' for each room," said Henry Rather, general manager of the Sheraton Bal Harbour.

To monitor the system and make the key-lock changes, Rather has assigned a full-time person. Learning how to make the key-lock changes takes perhaps a half-minute and requires no equipment.

"We can recode an entire floor in less than an hour," said Vilma Figueroa, who handles all the lock-system changes.

Changes in the cards and locks are usually made after checkout, unless there are more compelling reasons. Figueroa makes an inventory of room cards twice daily. If there are unaccounted-for missing card-keys, she replaces the lock and "keys" for the room immediately.

VingCard is just one of a number of key-lock systems being studied and installed by U.S. hotels. Some are mechanically operated, like the VingCard. Others are electronic or magnetic.

The Fontainebleau Hilton here, for instance, has just completed installing the Winfield lock system, also mechancial but employing batteries in the lock. This system, one of the more widely used in the country, uses a key with changeable plastic inserts rather than a card, and a separate key is used by maids. It also offers easy recoding of both rooms and master keys.

Under the Winfield system, there is a key-lock combination for each of the hotel's 1,200-plus rooms. But they can be interchanged quickly and simply, thus offering 1,200-plus possible combinations for each room.

Two persons work full time on the system under tight security conditions, according to Jerry Inzerillo, executive assistant manager. "Even the manager doesn't know how to program key changes," he said. Several other domestic Hilton hotels are also using this system.

ADT's Cardgard is another system in wide use. Guests are provided a card puched with coded holes and the system is operated electronically. It can also be programmed to turn off lights, television and other electrical appliances when guests are out of the room to save up to 12 percent on energy.

Other key-lock systems now being offered are Ellisson Lok, a battery-operated system combined with a plastic "key"; Marlok, a mechanical system; Schlage, which has a card but no slot -- you simply hold it near a sensor; Cardkey, an electrical system; Sargent's Restrict-A-Key, electromechanical; Russwyn Cardgard Reader System; Digit Key, which requires the operator to punch four digits to open the door; Digikey, a similar system; and Kenilworth FDA system, electronic, card-coded.

Use of the new lock systems is an accelerating trend, according to Ray Ellis of the American Hotel Association, and all major national hotel chains are taking a close look at them.

Most hotels are enthusiastic about the new systems, claiming they cut theft sharply. One hotel, for example, found that pilferage of bedding by its own personnel dropped dramatically after coded locks were installed. On the other hand, one large New York hotel reportedly scrapped a $1.3-million card-lock system because it did little to deter theft.

One hotel chain, Hyatt, has gone in another direction to achieve greater room security. Several of its hotels now have coded keys -- regular keys whose number does not correspond with the room number. If one is lost or stolen, there is no way anyone can use the key without knowing the code.