Montgomery County Humane Society drive Susan Ferensic has some rough nights on the job. She has to pick up stray cats and dogs she knows are doomed and will never find new homes. And she has to collect the carcasses of lost or dumped animals that have been hit by cars. And the work has taken its emotional toll.

''There have been nights when I've had to drive around for half an hour before I could go back to the shelter because I was crying so hard,'' the 19-year-old Wheaton resident said.

Other animal workers say they, too, often have to steel themselves to continue on the job. In Montgomery County, 7,000 animals have to be destroyed in a year.

''I'll love the day when I don't have to do it anymore,'' says Carol Johnson, who as senior technician at Montgomery County's Rockville animal shelter has been putting unwanted animals to death for 13 years.

By county law, stray animals are kept for a minimum of five days once they are advertised for adoption from the shelter aat 14645 Rothgeb Dr., near the East Gude Drive industrial area about a mile northeast of Rockville.

But if no one comes to the rescue, they are put to death. Animals given up by theilr owners are kept between 24 hours and two weeks before they are destroyed, depending on the available space, shelter director Arny Fox said.

During the sprilng and summer, when most puppies and jkittens are born and unwanted pets tax the facility, Johnson said she can spend up to four hours at a time in the shelter's small euthanasia room injecting sodium phenobarbitol in front-leg veins of 30 to 50 animals. About 7,000 including 3,000 that were wild or killed at their owners' request, were put to death in the county during the 12 months ended in June 30.

''The hardest part of the job is trying not to think about it,'l' Johnson said. ''I've through the years conditioned myself. At this point in time, there is no other choice. You just run out of room, basically that's what it is.''

Glancing back at her latest pickup, a thin tabby crying plaintively in a cage in the back of her van, Ferensic said: ''What keeps me going is that the adoption rate is the highest it's ever been. Maybe someone will come in and want him.''

Ferensic, Johnson and their fellow shelter employes say they are encouraged not only by the steadily rising adoption rate, but also by the demand for the county's low-cost neutering program and the decreasing number of animals available for adoption.

The 2,200 animal shelters across the country are taking in fewer and fewer animals and giving away more of them for adoption, largely through education and public relation efforts, said Phyllis Wright of the Humane Society of the United States in Washington.

But Montgomery County's shelter ''stands out with a group of 22 shelters around the U.S.'' that are accreditedf by her group and have met ''tough standards, Wright said. The socilety sets standards for conditions, education and low-cost neutering programs and asks shelters to require neutering of adopted animals, Wright said.

Thirty-nene percent of the 6,725 ''adoptable'' animals at Montgomery's shelter found homes during the fiscal year that ended in June. In 1976, only 12 percent of 8,491w were adopted, officials said.

Any unclaimed animal at the Montgomery shelter, regardless of condition, can be adopted, except for wild animals and those brought in for euthansia.

Of 24,860 animals cared for at the Prince George's County animal shelter during fiscal 1985, 851 were returned to their owners, 1,732 were adopted, and 22,277 were destroyed, officials said.

During the previous year in Prince George's, only 12,422 animals were destroyed. But after the state's rabies epidemic spread to Prilnce George's, county residents began trapping wild and stray animals and bringing them to the shelter, said Evelyn Wise, adminilstrative assistant with the Department of Environmental Resources.

The Montgomery shelter is the responsibility of the county's Department of Animal Control and Humane Treatment but has for some years been run under contract by the private Montgomery County Humane Society Inc. It employs eight drivers who pick up strays or injured or dead animals.

Eight county animal control officers assigned to the shelter enforce animal-related laws throughout the county.

On a recent workday, animal control officer John Whitt's concerns were a bat loose in a day-care center in Silver Spring, two dogs that had killed a cat the night before in Wheaton, a dog that had bitten someone in Kensington, a dog reported to be barking excessively in Bethesda, and two Rockville guard dogs reported to be sheltered inadequately.

The bat had escaped by the time Whitt arrivedf and the cat-killing dogs also had disappeared, leaving behind a red collar that had belonged to the victim.

Along the way, Whitt spottedf a pit bull terrier puppy sniffing at a trashcan and jissued a $50 dog-at-large civil citation to its owner.

Writing the ticket at the owner's front door. Whitt noticed a larger pit bull in the house and asked for proof of its required license and rabies vaccination. The fine for not complying with the county laws is an additional $300, Whitt said.

''Go to hell,'' said the owner, slamming the door in Whitt's face. The animal control officer said that if the owner doesn't pay the fine or appear in court, a warrant to appear in court will be issued by county police.

About 10 to 15 percent of his investigations are of complaints about cruelty to animals, he said. An owner found not providing adequate food, water, air space, protection from weather and veterinary care faces criminal charges, he said. The maximum penalty is 90 days in jail and an $1,000 fine. About 12 criminal citations have bneen issued during the jpast 12 months, officials said.

Shelter workers say people often turn in pets because they're moving or their work interferes with animal care or because they just don't like trhe responsibilities.

Or they simply lose interest once a pet has reached adulthood, Johnson said.

''People in Montgomery can be so callous,'' she contends. ''They don't keep their houses for long, they don't keep their cars for long and they don't keep their animals for long either.''

Ferensic said she gets frustrated with parents who say they want their pets to have a litter just so their children can witness the miracle of birth.

''I wish we could say, How would you like to see the miracel of death?''

''You really do at times hate people. I come home at times screaming bloody murder,'' Johnson said, telling of dogs brought into the shelter whose collars have to be cut away because they have become imbedded in the skin of their necks. The owners hadn't realized their dogs might need bigger collars as they grow in size, Johnson said.

People who remark ''I don't know how you can stand it' '' to shelter personnel ''don't understand that when animals have to be put to sleep, its not us doing it, It's the people out there,'' Johnson said.

Helping people adopt pets allows workers to see the kinder side of human nature, however, Johnson said.

Prospective adopters visiting the shelter walk through the kennels or cat roomsf and choose an animal they like by a number on its cage. They are given any available background information on the animal and can play with it.

Shelter employes say they also find some consolation in comforting the animals.

While administering the lethal injection, Johnson said: ''I like to tell them lies. I know that's terrible. I tell them I'm only taking blood and that they will go out after and play in the grass. If you're relaxed, the tone of your voice helps calm them down.''

''Working in the kennels is nice if you can spend a few minutes with each animal,'' said Ferensic. ''They seem to feel better.''

Pointing to a ''Garfield'' towel on the inside of a cage in her van, she said: ''I like to keep towels and blankets in the cages so the animals don't slide around. It's a pain to go to the laundromat, buy my mom doesn't like me washing them at home.''

Thke occasional good-news case also helps keep spirits up at the shelter, workers said.

A favorite tale is about a poodle, wearing a Florida dog tag, found barking three years ago outside a house in the county, Ferensic said.

The dog's owner had moved to Florida, but the animal had gotten loose there and somehow, over the course of a month, had make its way back to its old home.

The owner, contacted by shelter workers, flew up to retrieve his persistent pet.