Pack the mallets, Reginald, and saddle the ponies. We're off to Palm Beach to diddle away a chucker or two with the fancies who come in their family Ferraris to watch the gentry cavort on the polo fields.

No fewer than nine polo fields have been laid out in this settlement 15 miles across the Florida peninsula from the Gold Coast of Palm Beach. It is still in Palm Beach County, the fastest growing chunk of real estate in the nation, so the local tub-thumpers insist.

Each polo field is the size of 10 football fields, and with real estate on Palm Beach Island selling at a million dollars an acre (that's what the man said, honey) there is no way to lay out a polo playground in the midst of all the social hoo-ha.

So the developers went west and built the town of Wellington on 10,000 acres that weren't doing anything at the time. They left room for 6,000 residents to set up housekeeping and partitioned off 1,650 acres for a playland called the Palm Beach Polo and Country Club.

A grandstand was built on the main field, a festive tribune with striped awnings of yellow and white, lots of pretty cars and pretty people on Sunday game days. Last year even the Prince of Wales showed up to play and walked off with a silver cup. When the Argentinians come, the management sets up a barbecue in the style of the pampas with beef and ribs and sausages grilling on an open fire.

More decorously, one can have lunch at the polo club. Or swim in the pool. There is, to be sure, an 18-hole Fazio-designed golf course, with another one on the way. Bill Talbert runs the tennis club, which has 19 courts in play so far. A John Gardner tennis center is on the books for next spring.

If horses are your fascination there is stable room for 300 quadrupeds on the premises. Polo aside, horsemanship is practiced here in all its forms. There are show rings, steeplechase jumps, gymkhanas and dressage.

If you are only a biped there is a provision for you, too. The Palm Beach Polo and Country Club has built strings of low-rise condominiums around its sports facilities. At least 200 of the housekeeping apartments are in the rental pool. Figure $90 a day for one bedroom, $150 for two. After May the rate sinks to $60 and $110.

Half an hour's drive will bring you back to the old Palm Beach, which is being attacked on all sides by condominiums. The big buildings are coming up from Miami and Fort Lauderdale to the south. Condos have swallowed up many of the city's hotels, and builders are already sending towering piles skyward in West Palm Beach at the edge of Lake Worth.

If Palm Beach has a shortage of anything it is hotels, a matter that may be alleviated by spring when the Hyatt Palm Beach opens with 367 rooms. It will come completely equipped with a Regency floor (private keys to the elevators that slide up the outside walls) and an atrium as big as all outdoors.

Until that happy day P.B. is making do with a strange little Hilton that has been sandwiched into a tiny plot on South Ocean Boulevard. The tennis court is on top of the parking lot and everything else has been crammed into the main building, which affords glancing looks at the sea.

That great fortress, the Breakers, which has occupied its site in one form or another since 1895, has not been breached. Like most hotels in Miami Beach, it has lost most of its sand to a concrete esplanade sprinkled with lounge chairs for the comfort of the Roto-Broil people.

It has added an indoor pool for use when the weather doesn't behave, but golf is still played outdoors on its two courses. An outpost of the proper Palmy life, it still requires ties and jackets in the lobby when the sun goes down.

No one leaves Palm without an expedition to Worth Avenue which, until Rodeo Drive sprang to life in Beverly Hills, had no peer as the most fashionable and expensive shopping strand in the land.

It has recently added a double-decker mall. No K-marts here -- just a sprinkling of St. Laurent, Ungaro, Cartier and the centerpiece: Saks Fifth Avenue. Gucci commands a whole courtyard where it does a snappy business in Gucci loafers, which sell for $130 to $245. They are worn here without hose, probably because once you've bought the loafers who has lucre left for socks?