Dennis Day, the perennial "kid" singer whose signature line--"Oh, Mr. Benny"--was enough to generate thunderous studio applause on Jack Benny's radio and television shows, died Wednesday night of a degenerative nerve and muscle disorder. He was 71.

Day, whose hare-brained antics drove Benny to distraction for nearly 25 years, was critically injured in a fall in his Brentwood home in March. The accident ruptured several blood vessels in his brain.

Day, whose family took him home Monday from St. John's Hospital in Santa Monica after a 17-day stay, died at 10:20 p.m., said Day's daughter, Margaret McEniry.

McEniry told United Press International that she and Day's nine other children were with him when he died. However, his wife, Peggy, was hospitalized Wednesday morning after suffering a mild heart attack. She was informed of her husband's death and was expected to be released from St. John's Hospital today, McEniry said.

Nerve Disorder

Last year, doctors said Day was suffering from amyotrophic lateral sclerosis, a rare, fatal nerve disorder commonly referred to as Lou Gehrig's disease.

A mischievous leprechaun of an Irish tenor, Day was also a talented mimic who would bury Benny beneath a cascade of dialects and characters, only stopping when the exasperated comedian would sigh and say, "Oh, for heaven's sake. Sing, Dennis."

And sing he did in his smooth, lilting tenor. His selections ranged from "Too-ra-loo-ra-loo-ral, That's an Irish Lullaby" to the lively, foot-tapping "MacNamara's Band."

A Native of New York City, Day intended to enroll in law school at Fordham University after graduating from Manhattan College in 1939. But a recording of his songs that he mailed to Benny's wife, Mary Livingston, changed those plans permanently. Livingston persuaded her husband to give Day a 2-week tryout.

"I was a green kid out of college when I joined the show Oct. 8, 1939," Day said. "I'll never forget it. I was scared stiff. But he (Benny) was like a father to me, so kind and gentle. Two weeks went into practically 35 years."

'Green Kid'

To Benny, however, Day remained that "green kid." When the singer was well into his 40s, Benny still called him "the kid."

A devout Catholic, Day married Margaret Ellen Almquist in 1948. The couple had 10 children.

In interviews in the past few years, he would smile uncomfortably, and concede, "I have a tough time remembering the kids," when he could not recall his married children's names.

Day was born Owen Patrick McNulty. He legally changed his name to Dennis Day in 1944, only to change it back to McNulty in 1947, "just to keep the Ancient Order of Hibernians happy."

Despite the lame-brain character he played on the Benny shows, he was an astute businessman. Scrutinizing NBC's scenery budget for his own show in 1953, he decided to build his own sets and save the difference.

The network argued that transporting the sets would offset the savings, but Day bought a building near the theater where the show was produced. He put two stores in front. Rentals repaid his investment, covered maintenance and provided a small profit.

Silver Dollars

One of his hobbies as a child was collecting silver dollars. In 1951, he signed a contract to sing at a Reno night club, and he included in the fine print that he be paid in silver dollars.

Although he specialized in Irish ballads like "Danny Boy," and appeared in eight films, Day was best known for his playful encounters with Benny.

"There was an empathy between us," Day said.

The comedian showed up at Day's home on Christmas Day, 1957, wearing a Santa Claus outfit, beard and all. The disguise almost worked, Day said, until Benny pulled out his violin and began to play.

Benny's radio show started in 1932 and remained on the air until 1954. He continued on television for another decade after that. The singer and comedian remained friends until Benny's death in 1974.

In recent years, Day sang at conventions, fairs, churches and clubs. He continued to make appearances at benefits, despite the fact that he was confined to a wheelchair.

Encore Performance

In 1985, he teamed for the first time with his sister-in-law, Ann Blyth, at an appearance in Downey. They were such a hit, that they agreed to an encore performance at El Camino College in Torrance.

Among Day's popular recordings were "Peg of My Heart" and "Clancy Lowered the Boom." His films included "Buck Benny Rides Again," "Music in Manhattan," "One Sunday Afternoon" and "Melody Time."

Owen patri C Mcnulty