A haggard looking Margaret Smith emerged from her Fairfax County home last night and faced a lawn crowded with reporters and lit by television floodlights, "I've heard from my boy and he's all right. He sounded OK," she said. "Tht's all I have to say."

With that simple announcement, law officers from four agencies ended a massive 20-hour search for 1o-year-old Anthony Leto, a quiet Astudent who interrupted his evening paper route without explanation to catch a train to Toldo. He left behind parents frantic whit worry, 42 police officers who with police dogs scoured the woods and storm sewers near his home and his parochial school classmates who attended a special mass for his well-being.

Almost 20 hours after Margaret Smith of 4612 Kling Dr. in the Lincolnia Hills subdivision, frantically called Fairfax County police to report her son missing, her telephone rang. It was leto, calling from a Toledo bus station, saying he was fine after traveling to Ohio "on the spur of the moment."

Although Lated that her son was not -- as polie had feared -- a victim of foul play, Smith said last night he would face stern discipline when he returns home. She said Leto would be expected to pay the transportation costs of Ohio and Colorado relatives who flew to Washington when they heard he was missing.

Furthermore, she said, Leto will have to work as a volunteer with local ploice departments to repay them for the costs of their search. At one time, Fairfax, Alexandria and U.S. Park Police, and Alexandria sheriff's deputies were pressed into the effort to find him.

"I cried years' worth of tears in the past 20 hours," Smith said. Her son, she said, didn't realize "what everyone had gone through" since he left Washington after collecting money from some of the customers on his paper route. He paid for the trip himself, she said.

What had troubled police about Leto's disappearance was his reputation as a dependable, responsible youth. Friends and acquaintances said earlier in the day that they could not believe that Leto would leave his home without explanation.

Barely a hour before her son called, Smith, a divorcee, appeared before reporters and cameras to make an emotional plea for help in locating her son. "I want my boy back," she said softly. "He knows I love him and am very proud of him. We're just waiting to hear."

Police theorized that Leto had gone to Toledo to visit his grandmother there. But Smith said later that her mother was one of the relatives who had come to her home when Anthony, the eldest of her two sons, disappeared.

She said last night that the family hoped to fly him back to Washington, but that the trip had been delayed by an ice storm that hit Toledo last night.

Police, who searched throughout Monday night for Leto in subfreezing weather, said last night they had no complaint about the end of the case. "We're happy because most of the time they don't turn out like this," said Fairfax investigator Colin Kozloff. "Most of the time they turn up as bodies in the woods. We're just overjoyed."