Out at the U.S. Customs Service training center in Front Royal, a recent graduate prepared to go through his paces. His eyes shone with eagerness and, it being a hot day, his tongue lolled outside his mouth.

Just what you'd like to meet at the end of the journey.

He jumped on a conveyor belt and eyed the sample packages coming through, merrily cuffing aside those marked "FRAGILE" or "FIRST CLASS" or

HANDLE WITH CARE." Then, to the delight of his instructors, he pounced on a package and began tearing it apart.

The offending package turned out to have marijuana in it. If it had been yours, you would probably not wish to make a fuss about the way it had been handled.

However, suppose yours had been one of the two through which the inspector had accidently stuck his foot on his tour, or one of the two innocent packages which he had mistakenly torn apart with his teeth?

That would never happen in a real situation," instead Gene McEathron, who developed and supervises the drug-sniffing customs program. He was proud of the performance of Lucky, the over-zealous inspector,and was ready to vouch for the idea that Lucky would confine his rougher activities to drugged packages if he had been working a regular day.

Lucky, and 100 other dog-trainees at Front Royal, are expected to follow the regulations. On the grass in front of center headquarters is a neat sign, "Attention Dogs: Do Not Poo-Poo on This Lawn."

"They love the work - they don't work for food, they work for the satisfaction of it," said McEathron, who pleasantly informed Lucky that if he didn't measure up, he'd be retired from the service. A visiting representatives from the Humane Society confirmed what he said about the dogs' well-being.

McEathron had been training patrol dogs for the U.S. Air Force when he was asked by the Customs, three years ago, to develop dog drug-inspectors. "Some police departments had tried it, but there was nothing to evaluate their programs by; the Army had started a program, but dissolved it," he said. So he began his own in 1970 with 10 dogs and some heroin.

He now has 128 dog-inspectors in 48 different ports, detecting hard and soft drugs in freight, luggage, mail, warehouses, cars, airplanes - just about every place a human inspector would check, only faster. A dog can do a complete car - engine, trunk and underneath included - in five to seven minutes, while a person would take 20 minutes to do a spot check.

There are five different programs offered for dogs and their trainers, including one in explosives detection. The plan is to have 500 dogs in the field by 1982.

"We're an equal opportunity employer," said McEathron. "We take males, females, all colors." Height, weight or good looks don't count.Health does.

Family connections count a lot. So far, only one out of every 43 applicants has been accepted, but of the dogs being bred on the premises - there is a fifth generation of 29 "perfect puppies" bred for the job - 75 per cent are expected to make it."

An aplicant is first tested at the age of 5 weeks. What can you tell about him? "What can you tell about a two-month-old child?" asked McEathron. "Not a hell of a lot."

The inspectors look for "a submissive attitude" - that's what they don't want. "We want one that'll fight back."

At 8 weeks, the puppy is sent to a foster home, usually that of a Customs inspector who expects to work with him, but sometimes into the community. "We took for a place that has a fenced-in yard, and we like to have a family with children. The puppy needs to be teased and petted, and to run into things you routinely see in a home." The center provides food and medical expenses, and shows the foster family how to play retrieving and tug-of-war games with the dog.

At the age of a year, the dog is again inspected - for such characteristics as a "a degree of self right," or "the conviction that he has a right to the space he occupies - and failures become the property of their foster families. (Yes, they have had emotional problems with families whose dogs have passed and therefore left home, but "nothing will mend a broken heart as quickly as a new puppy.")

The training consists of "reinforcing the dog's natural love of retrieving and tug-of-war" and associating it with the smell of drugs. Wrapped towels, which the dogs use in these games, are kept overnight in lockers which contain drugs.

The game is made more and more complicated by having the drug-smelling towels hidden in open spaces, truck hidden, and school buildings. (Exercises in the Warren County junior and senior high schools have turned up more drugs than the trainers had planted there.

"There are treats in this program, and no punishments - these dogs are working because they enjoy it," said McEathron. "The dog that graduates from this center has no idea he's working - he thinks he thought up the whole thing."

In training, the dog is allowed to destroy the drug-smelling containers, so that he can extract the towel and play with it, which is the motive behind his service to his country.However, in situations, he is not supposed to go around destroying the mail - a trainer stands ready to grab from him whatever packages he points out by his interet.

"Once he scratches a box, his job is over," said McEathron. But to keep him interested, dummy packages with drugged towels are inserted into his inspection area occasionally, and he is allowed to tear them apart.

And after all, there's no incentive like being able to rip apart other people's packages.