Leonard Walls sat ginerly on the only chair he owns and stared in amazement as the huge chips of corroding black, brown and gray paint were scraped from the ceiling and walls of his one-room apartment.

He watched the refuse fall onto the remains of two decomposing mice, onto his bed piled high with his few possessions and deep into a corner rat hole, causing a score of roaches to scatter.

And, above the scraping noises and the squeaking of paint rollers, Walls listened with growing pleasure to sounds he hadn't heard in years - the songs and laughter of teen-agers busy around him.

The youths working in Walls' tiny home at 425 M St. NW, as well as a dozen other members of the Gaithersburg Presbyterian Church Youth group, gave up the first week of their summer vacation in order to paint and haul trash to improve the living conditions of Washington inner-city residents they'd never before met. One of the young workers took a week's vacation from his job to participate.

"We did it to serve the Lord," echoed several members of the group.

They commuted to Washington each day as part of a week-long "inner city camp" organized by the Presbyterian Church to give suburban youth groups a chance to experience urban ministry. The Rev. Charles Summers and his wife, Marsha, of the Sixth Presbyterian Church at 16th and Kennedy streets NW, organized the project.

After a Bible study session at Sixth Presbyterian each morning, the high school and college students visited inner-city centers like the House of Ruth (a shelter for homeless women), Jubilee Housing and SOME (So Others Might Eat), all projects motivated by religious concern for the poor.

In the afternoons, the young people and other volunteers met at the Urban Ministry headquarters near 3rd and M streets NW. Urban Ministry, funded by local and national Presbyterian churches as well as businesses and private group, strives to improve the living conditions in the immediately surrounding inner-city neighborhoods.

The church group helps local tenants by locating medical and legal help, nudging landlords to improve properties and organizing work crews like the Gaithersburg group to do heavy upkeep on apartments and houses.

Equipped with paint brushed, ladders, buckets and portable radios, the cheerful teen-age troops pair off to work in different dwellings in the neighborhood.

Although at first many area residents seemed untrusting or unfriendly toward the young people, by the end of the week the teens and tenants reached an easygoing rapport with each other.

After following the workers around from house to house, some neighborhood children soon became adept at finagling potato chips and piggyback rides from the teens.

Some of the Gaithersburg group, used to comfortable suburban living, said they were shocked at the living conditions in the neighborhood. "We saw things this week we never saw before." said one girl, as the paint-spattered group gathered to leave at the end of a workday.

"I wasn't really expecting such bad conditions," said Kathy Ruggles, 14, "It was a learning experience. I was brought up middle-class and never experienced anything like this before."

"We got first-hand experience what poor really means," said Paul Henry, 20. "You can see it on TV or read about it in the [newspapers], but it's not the same as seeing it."

The Rev. Bert Moore, director of the Gaithersburg group, said: "I've watched them grow this week. You can't lecture from the pulpit on something like this."

Most of the group said they hope to participate in the program again next summer.

"At the end of the day I'm tired," said Cindy Moser, 16. "But I feel really good inside. I feel I made a difference." CAPTION: Picture, Randy Ruggles scrapes paint and plaster in room of Leonard Walls. By Joel Richardson - The Washington Post