WASHINGTON (Reuters) - Hsing-Hsing, a giant panda given to the Smithsonian National Zoo in 1972 to commemorate President Richard Nixon's historic trip to China, died on Sunday after suffering from kidney failure associated with old age. The panda was 28.

``It is with great sadness that we report the death of Hsing-Hsing,'' Friends of the National Zoo Executive Director, Clinton Fields, said in a statement. ``Our beloved giant panda...suffered from the complications of kidney failure associated with advanced old age.''

Hsing-Hsing fell ill with testicular cancer two years ago but recovered. In recent months the panda suffered from kidney failure, a condition which grew sharply worse in recent days leading zoo officials to put him down on Sunday morning after it was decided he was no longer responding to treatment.

``Just as your pets are part of your family, Hsing-Hsing was part of ours, and most especially part of the lives of the keepers who took care of him and the volunteers who watched him over the years,'' Fields said. ``Parting with such a cherished animal was difficult for us, but humane euthanasia was in Hsing-Hsing's best interest.''

Hsing-Hsing, one of the longest living pandas ever, came to Washington along with another panda, Ling-Ling, on April 16, 1972, as a result of Nixon's visit which opened diplomatic doors with China. The panda couple became a symbol of friendship between the two nations.

The pair were an instant hit at the zoo with more than 20,000 people visiting them on their first day with millions more visiting them in the following years.

The pair mated many times but never produced any offspring. Ling-Ling died in 1992 of heart failure.

In recent months Hsing-Hsing supplemented his usual diet of bamboo shoots with a diet of blueberry muffins in which zoo keepers concealed his medicine.

As Hsing-Hsing grew sicker during the summer the popular panda was often absent from his glass enclosure. His illness drew sympathy from across the country with animal lovers and school children flooding the zoo with well wishes.

Hsing-Hsing's death leaves the National Zoo without a panda. The zoo has negotiated with Chinese authorities about leasing pandas but has so far failed as the price of pandas escalates.

A spokesman for the zoo said he remained ``optimistic'' that the Chinese would accept the zoo's offer of $2.5 million for a 10-year loan of two pandas. He said the zoo's offer was more focused on its research and training capabilities rather than the cash offer.

Earlier this month two giant panda cubs from China arrived at an Atlanta zoo. The two-year-old cubs, a male named Yang-Yang and a female named Lun-Lun, were brought to Atlanta on a 10-year loan at a cost of $1 million a year.

While Zoo Atlanta was able to fund that fee by raising its admission price to $12 from $10, the National Zoo has difficulty matching that size of offer since it has no admission fee.

In the wild pandas live about 15 years, but in captivity they can live well into their 20s.

The giant panda is one of the most critically endangered species and its numbers have fallen sharply in recent years. There are fewer than 1,000 in isolated groups in western China: only about 120 pandas live in zoos around the world.