A drug manufacturer has urged veterinarians to reinoculate dogs that were given its best-selling brands of rabies vaccine because the animals may not be protected.

No one knows how many dogs are affected but the manufacturer of the vaccine, Norden Laboratories of Lincoln, Neb., says it has about one-third of the national market. Maryland health officials, who yesterday urged dog owners to heed notices from their veterinarians, estimate that between 400,000 and 800,000 dogs in that state may have received defective immunizations.

"This is a public health issue and we want to make sure dog owners take their veterinarian's advice if they get notices saying their dogs should be brought in," said Michael Golden, a spokesman for the Maryland Department of Health and Mental Hygiene.

Maryland led the nation in rabies cases in wild animals, mostly raccoons, until 1986, when Texas edged ahead. Contact with these animals is the main way dogs contract rabies. Although only three cases of dog rabies have been reported in Maryland this year, officials believe many dogs exposed to rabies were destroyed before they could develop symptoms.

Not all dogs given the vaccine are at risk because there are two ways it can be administered and only one failed a government-conducted test, Norden officials said. The vaccine can be injected into a muscle, the established method that is still considered effective, or subcutaneously, just under the skin, the newer procedure that failed.

The two vaccines manufactured by Norden -- Endurall-K, said to be good for one year, and Rabguard-TC, a three-year version -- had been sold for intramuscular injection for many years and had proven effective when given this way. In 1985, however, Norden obtained provisional government approval to tell veterinarians that they could also use the subcutaneous method.

Subcutaneous injection is considered less painful to the animal and preliminary tests showed that 30 dogs vaccinated this way produced just as many antibodies against the rabies virus as did those given intramuscular injections. The dogs, however, had not yet been exposed to the rabies virus to see whether they really were immune.

Last month, at the end of a three-year waiting period, the vaccinated dogs were exposed to the viruses and about half contracted rabies, far below the minimum of 86 percent protection required by the U.S. Department of Agriculture, which regulates veterinarians.

Norden then sent letters to veterinarians recommending that they no longer give subcutaneous vaccinations and urging that affected dogs be revaccinated.

The vaccine also was given to cats by the subcutaneous method but a Norden spokesman said the results of those tests are not known yet. However, the company is telling veterinarians not to give any more subcutaneous vaccinations to cats.

The dogs most at risk are those that had never been vaccinated and those that have received only one dose since July 1985. Such dogs should be revaccinated as soon as possible, Norden's president, Daniel B. Hinnah, said in the letter. The reason is that successive immunizations tend to act as boosters, building on previous effectiveness, but the first immunization has nothing to build on.

The second highest risk is to dogs last immunized between July 1985 and August 1986 that have had more than one subcutaneous dose.

Officials of the Maryland Veterinary Medical Association said yesterday that when Norden recommended the subcutaneous method, veterinarians were not told the government approval was only provisional. The package had said that tests done under federal regulations "demonstrated that a single . . . dose protected either dogs or cats satisfactorily."

In fact, the vaccine's ability to protect by the subcutaneous method had not yet been tested.

Norden's director of customer affairs, Cecil Metzger, said that when the subcutaneous method was added to the directions, the claim of protection that had applied to the intramuscular method was not deleted.

"I can see how they {veterinarians} would have misunderstood," Metzger said. "The whole label should have been redone."

Metzger said the company is offering free replacement vaccine and syringes to veterinarians and working out a method to compensate them for administrative costs.