Excellent new videotapes offer advice on buying dogs and altering the behavior of the animals.

In "A Day in the Life of a Dog" ($29.95, Nu-Sounds, 40 Exchange Place, Suite 1914, New York 10005), Yorkshire terrier fancier Arlene Klein warns buyers to "consider what your responsibilities will be. Are you ready to invest the time and meet the challenge of dog ownership?"

Klein emphasizes that dogs are social animals that need to share family life. The scenario moves smoothly through a varied day: The pets go to a marina, romp at home, participate in conformation shows and puppy obedience classes and enjoy being groomed. There is important emphasis on use of crates to transport dogs and gates to keep them where they should be.

One flaw: In addition to being shown Klein's beautiful home and Yorkies, viewers should have been introduced to an occupant of a relatively low-income apartment, and caring for the animal before and after work.

One of the best features is an extensive bibliography of books on dog acquisition, care and training. A portion of profits from this tape goes to the nonprofit Morris Animal Foundation.

If the dog you're thinking of buying is a German shepherd, an indispensable aid to puppy choice and care will be "How to Select a German Shepherd Puppy and Develop a Champion" ($29.95, Canine Film Corp., P.O. Box 262, Summerfield, Fla. 32691).

Though aimed at buyers of show dogs, the person seeking a shepherd for a pet also needs this common-sense advice on how to find a reliable breeder, identify genetic problems and come away with a healthy pup. It is the work of Jane Bennett, Cappy Pottle and Gloria Birch, all present or former board members of the national German shepherd club. There is a lot of good information here on daily care, training and housebreaking.

A tape by well-known dog trainer Joseph Knapp could be a lifesaver for owners of dogs that do such things as chew everything in sight, steal, dig up the garden, jump on people or can't be housebroken. "Solutions to Problem Behavior in Dogs" ($29.95 plus $3.50 shipping, Joseph Knapp, 23 Essex Rd., Bethpage, N.Y. 11714; 516-433-1960) offers specific directions anyone can follow.

Knapp's system is based on a dog's natural instincts. He warns that if you isolate your dogs, which are pack animals, from the family, which substitutes for the pack, the pets will develop negative behavior. He advises keeping your dog near you on its lead, even around the house, so misbehavior can be corrected constantly and immediately.

Knapp covers what to do when your dog digs in the garbage can, jumps up to steal food from the counter, climbs on furniture you don't want it to soil, turns your yard into an archaeological dig and dozens of other infractions.

"If you can keep the dog doing the right thing for a week, you can change its behavior pattern," he promises. "The longer you correct, the less likely the dog is to misbehave. Don't dog-proof your house. Train your dog."