The National Zoo's giant panda cub is a "solid little Tonka truck," the animal park's chief veterinarian said after examining him yesterday morning.

The next big step in his development will be when his eyes open, and Suzan Murray said it appears that will happen soon. The cub responded to a tap on his eyelid by crunching his lids and trying to open his eyes, which she described as "the beginning of a blink reflex."

He also had a ridge along his gum, she said, but teeth will not erupt for several weeks. His tummy was round, she said, and heart and lungs sounded good.

The cub, born July 9, will be six weeks old tomorrow. Yesterday's checkup, which began at 7:33 a.m. and lasted 14 minutes, was his third. The cub and his mother, Mei Xiang, were more relaxed about this exam than they were about the previous ones, Murray said. The cub did not squawk during the exam, and "we were saying he almost looked asleep," Murray said. A check with a measuring tape showed him as 17 inches long; he weighed more than four pounds.

His mother was eating bamboo in the next room of the Panda House when keepers closed the door to the birthing den and took the cub. She noticed the sound of the electronic door, Murray said, but did not react. During earlier exams, she went over to the door and pawed anxiously. After the door reopened this time, she continued eating for several minutes before returning to her cub.

Mei Xiang did not leave the cub for five days after his birth and stuck so close to him that veterinarians were unable to examine him -- and determine his sex -- for more than three weeks. Now, though, the mother usually leaves her cub by himself in the den four or five times a day while she goes to eat in the adjacent room of the Panda House. After cradling him in her paw for his first several weeks, when the helpless newborn did not have the fur to keep himself warm, she now often rests by herself a few feet away from him.

Murray, who had not participated in the two earlier checkups, said she was impressed by how solid the cub felt.

"It just feels great in your hand -- it fills out the palm of your hand perfectly," she said. She described the cub as "the opposite of frail. You can sense that by looking at him. He is a solid little character."

At birth, panda cubs weigh about four ounces and measure a few inches long. At this rate, Murray joked, veterinarians need to do his next exam soon "or he will double in size" before they can. The measurements will go into a database of captive panda statistics to be used for research.

The cub is the first for Mei Xiang and her mate, Tian Tian, who arrived at the zoo in December 2000 on a 10-year loan from China. Under the agreement with China, the cub is the property of that country and is to return there after his second birthday. The zoo is paying China $10 million in privately raised money for the pair and $600,000 for the cub. The money is to go to conservation projects to save the endangered animal in the wild.

The cub will not be named until he is 100 days old, and Chinese officials will choose the name. The Panda House is closed until October, but Tian Tian can sometimes be seen in the outdoor yard.