LAGUNA BEACH, CA - It doesn't attract gawkers in this hilly southern California beach city as we tool down the main drag, Highway 1, some 45 miles (72 km) from Los Angeles.

That's because outwardly it doesn't look much different than a standard 2000 Buick Regal. And you wouldn't mistake it for a typical convertible with the top down, even though it is indeed a "convertible." From the side it looks like any other car.

But this is no ordinary family sedan. Called the Regal Cielo (Spanish for "sky"), it's a prototype of a "topless" midsize Regal that debuted at the Greater Los Angeles Auto Show last month and elsewhere on the show circuit later on.

The last 4-door convertible was a Lincoln that bowed in 1963.

Jaguar also featured a cloth-top sedan way back when. Buick will gauge consumer reaction to the idea, which its management strongly supports. But right now it's under a yellow "caution" flag with numerous engineering challenges still ahead. If it indeed gets a green light, look for it to reach market in 2001, perhaps with a $5,000 premium.

"We should get strong answers during the next three months," says Mike Doble, manager of specialty vehicles. Even then "we will still need to complete feasibility studies. But because we're working with a regular production vehicle we can do things relatively fast."

By "topless" we mean the conventional roof disappears into the trunk compartment at the push of a button, leaving an open-air 4-door sedan for checking out the ambient temperature, sunset or moonrise. There are two other options to the system as well: It can work as a sunroof spanning to each roof rail, or it can be used to lower the rear window.

Buick hinted at where it might be going when it unveiled the radically designed Cielo concept car at the 1999 North American International Auto Show in Detroit.

The Regal Cielo, however, bears little resemblance to the Cielo show car - except that they share the name and both have electrically powered disappearing roofs.

In the Regal Cielo, two movable roof panels slide rearward and are stored, along with the rear window, vertically in the trunk behind the back seat. They enter via a hinged tonneau cover located behind the rear window that flips open as the panel and window enter, then shuts to its original position.

The roof can be lowered while the car is in motion, something you wouldn't want to do in a conventional "ragtop."

The panels and window reduce usable trunk space by 40% to 9.5 cu. ft. (269L) from the standard 16.7 cu.ft. (472L), still enough for substantial cargo.

Although the Regal's overall length remains the same, the top mechanism and peripheral items add 109 lbs. (50 kg), says Mr. Doble.

Regal Brand Manager Mark D. Hines says Buick clinics indicate buyers want the safety of four doors and strong upper structure in convertibles.

Mr. Doble's group is working on durability and safety issues such as beefing up the windshield header, adding lateral structural beams, providing more support at the rails and, possibly, suspension changes, he says. Other issues include wind noise, water control and security measures.

Buick sold about 75,000 Regals in 1999. Mr. Hines won't say what volume might be generated if Cielo gets the go-ahead. "We can go for higher volume ourselves (with Cielo) or farm it out (to a specialty shop)," he adds.

To set it apart from regular-production Regals, the Cielo prototype has special 17-in. (43-cm) wheels and tires with Baer racing brakes; a two-tone pearlescent platinum three-coat paint scheme, special lower rocker panel cladding and a specific body-color grille with air intakes for cooling the front disc brakes.

Cielo badging includes a nameplate on the right rear taillamp and the name embroidered on both front door panels and on the passenger side of the instrument panel.