As she flew into exile at President Reagan's Santa Barbara ranch, former First Canine Lucky yanked her master toward the press section on Air Force One. Since the 65-pound sheepdog was known for rambunctious spirits, flying leaps and what the White House delicately termed a "puddling problem," the reporters became a bit skittish. The President noticed their apprehension. "I get good reviews," he said, "or I turn her loose."

Sic transit Lucky. Such threats can't be made with her proper, 16-pound replacement, Rex. A Cavalier King Charles Spaniel, Rex counters Lucky's down-home scruffiness with Ivy League chic. For example, the pampered 1 year old often sleeps in a pillow-lined basket at the foot of the Reagans' bed, a place where Lucky was never allowed to tread. Rex is also the first presidential pooch to undergo a tonsillectomy. And his pedigree is so exclusive that breeder Irene Murphy, of Greenwich, Conn., insisted on interviewing the Reagans before allowing them to take Rex home.

Rex's behavior matches his background. According to one reporter, Lucky was "as obedient to the President as the House of Representatives." In contrast, Rex is impeccably mannered and has no difficulty with what Elaine Crispen, Nancy's press secretary, calls his "necessary duties." While the hapless Lucky roams the 688-acre ranch out West, Rex is content spending "the bulk of the day in Mrs. Reagan's lap or at her side," reports Crispen.

Order has thus been restored in the White House, and yet a kind of void is felt. Staffers speak fondly of Lucky's "verve and personality" and miss seeing her "pulling Big Mac trash from the cans." Even the President is pining. Asked about Lucky, Reagan said he's planning to discuss the matter with his staff. "I'll talk 'em into taking more trips to California."