Teddy Ruxpin, the popular talking toy bear, is sick.

Just how sick is unclear, but there should be no cause for concern, according to Dr. Bearwell in the land of Grundo where Teddy lives.

In a letter from the toys' maker to Teddy's unhappy friends, Dr. Bearwell notes that a special Grundo General Hospital has been set up "to take good care of him and send him back as soon as possible after he arrives. . . . Teddy Ruxpin will be back before you know it."

The unusual rehabilitation program has been started by Worlds of Wonder Inc., the company that put Teddy under thousands of Christmas trees last year. The rage of Christmas 1985, has become the outrage of 1986 for some retailers who report an unusually high number of returns of the highly popular tape recorder in bear's clothing.

The problem, it seems, is not in its fancy mechanical and electronic parts that make his nose and mouth move to the music, but rather in the tape recorder itself.

"There wasn't any problem with animation; that was great," commented John Hall, store manager of Lowen's, a toy store in Bethesda. "But the tape often got stuck or wouldn't want to play when the door was closed," Hall said. "We got back about 20 percent" of what the store sold.

Evans Distributors & Jewelers Inc. also reported a 20 percent return rate. Other stores, however, had smaller problems. Bradlees said its return rate was 6.2 percent. The Hecht Co. reported that it received complaints on only 3 percent of the bears it sold. And Toys R Us, the nation's largest toy store chain, while not releasing a specific number, said the return rate was "in line with that for other electronic products."

No matter how large or small the ailment may be, Worlds of Wonder Inc., is taking no chances. Not only has it redesigned the product to eliminate the infirmity, but also it has launched a "Kids Happiness Program" to patch up any ailing Teddy.

Under the program, retailers have been given a special letter to hand over to any disgruntled customer. The letter from Dr. Bearwell directs Teddy's friend to mail the bear to Grundo General Hospital at Worlds of Wonder headquarters in Fremont, Calif. Within a few days of receiving the patient, Worlds of Wonder will send a new, improved teddy to cuddle and play with.

"The letter is unprecedented in the toy business," acknowledged Bob Goldberg, Worlds of Wonder's executive vice president for marketing. But, he added, that did not mean that there was a serious defect problem.

"We estimate that the problem overall is very small -- about a 3.5 percent defective rate," he said. Having sold a million bears, that means there are probably 35,000 with a problem, a very small percentage for any new toy, Goldberg said.

Despite its current woes, toy industry officials remain bearish about the young Mr. Ruxpin.

"Teddy Ruxpin has captured the imagination because of everything Teddy Ruxpin is," said David S. Leibowitz, a financial analyst who specializes in the toy industry for American Securities Corp. "I don't believe Teddy Ruxpin will lose its popularity."