Q. My 8-year-old daughter is getting her second set of teeth, and they are all coming in with a very mottled appearance. What do you think could be the cause of this?

A. Several conditions can discolor teeth in childhood. The antibiotic tetracycline can cause a grayish or brownish discoloration in children exposed during late pregnancy or early childhood -- up until about 8 years of age.

People consuming large amounts of fluoride develop darkened teeth. Interestingly, this effect served as a clue leading to the discovery of fluoride's anticavity action. Researchers traced the source of a community's tooth discoloration to very high levels of natural fluoride in the drinking water. The investigators coincidentally noticed that the local inhabitants had remarkably low rates of tooth decay. This effect does not occur in communities where a controlled amount of fluoride is added to the drinking water but may happen if well water has unusually high amounts.

Severe attacks of childhood infections, such as whooping cough or measles, may lead to patches of tooth discoloration. There are also some rare diseases, such as porphyria and ochronosis, that may discolor teeth, but I wouldn't expect a change in tooth color to be their only sign.

Finally, if a tooth dies -- from decay, trauma or for no apparent reason -- it will eventually turn slightly gray. But this explanation would not account for an entire set of teeth becoming mottled.

If the discoloration is superficial, your dentist may be able to treat it by polishing. For deeper discoloration that's very unsightly, you can have white porcelain or plastic caps permanently bonded to the teeth, or have them covered with a white synthetic veneer.

Q. For several years, I've had a strong urge to get up during the night to urinate, sometimes as often as every two hours. During the day, I'm fine and can wait four or five hours between trips to the bathroom. I've seen my family doctor, who tested me for infection but didn't find any. He sent me to a urologist, who enlarged the opening to my urethra, but that didn't help. What could be causing this? I'm a 70-year-old woman.

A. Getting up at night to urinate -- something doctors call nocturia -- has many possible causes. But not all nighttime trips to the bathroom are a sign of something wrong. Many people regularly get up once or twice a night, a pattern that's normal for them. The clue to something being amiss is a recent or distressing change in your usual routine.

Since you're troubled by these nightly awakenings, I'll mention a few possible causes and discuss ways to look into your problem. Kidney disease. Several types of kidney disease make it difficult for the body to hold on to water, which seems to pass right through you. Doctors use blood and urine tests and kidney x-rays to check for these kinds of problems.

Bladder disorders. Infections, inflammation, tumors, stones and blockages in the passage of urine can cause nocturia. Your urologist probably already checked you for these conditions with a procedure called cystoscopy -- examining the bladder through a narrow tube inserted through the urethra, the channel that empties the bladder.

Prostate enlargement in men can constrict the urethra and lead to frequent nighttime trips to the bathroom. Because of the partial blockage, the bladder never fully empties, calling for return visits to the bathroom. Diabetes can make you pass large amounts of sugar and water in the urine. Frequent urination is an early sign of this condition, which shows up on a simple blood or urine test.

Diabetes insipidus, a rare disorder unrelated to ordinary diabetes, makes the kidneys unable to retain water, with large volumes passed each day. A few drugs, such as lithium, can weaken the kidney's ability to hold onto water, leading to a similar result. Alcohol, known for causing frequent voiding, works in much the same way.

Diseases that make you retain fluid, such as heart failure and cirrhosis of the liver, lead to increased voiding at night. After you lie down in bed, the fluid that collects in the legs during the day enters the circulation and passes into the urine.

Problems with the nerves of the bladder can trigger frequent urination, a condition sometimes called spastic bladder. Tests of bladder function by a urologist can pinpoint this disorder as the culprit.

Most of the problems I've mentioned would also give you trouble during the day. Because you're bothered just at night, it might be that you have daytime fluid retention from something like early heart failure. Another possibility is that you have a mild case of spastic bladder and are able to cope with it during the day. At night, when you're more aware of bladder discomfort, you feel compelled to get up.

To look into your problem further, I'd recommend asking your doctor about tests of bladder function. One new resource for women with problems with incontinence and bladder problems like yours is the Continence Center at Columbia Hospital for Women (293-5211).

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.