QUESTION: Where do you find a good head lice remedy? We can't seem to get rid of our son's case despite liberal use of the store-bought stuff. He's still itching a week later. - Mrs. R.R.

ANSWER: There are many effective head lice remedies, some requiring prescription. Most of them work well given a chance. I would trust the pharmacist to recommend the one his customers have found most effective. Here are some important tips in treatment:- Fine-tooth comb the hair after applying the preparation. This removes lice eggs, which are unaffected by the shampoos.

- Repeat the treatment in seven to 10 days. That kills any emerging eggs that might have missed the first assault.

- Wash clothing and bedding in hot water. Dry on a hot dryer cycle.

- Put clothing that can't be washed in a plastic bag for 10 days. Lice cannot survive off the body longer than 48 hours.

- Disinfect combs and brushes with alcohol.

- Consider treating other family members as a preventive measure.

- Check with your doctor to be certain that a lice product, including shampoos, can be used safely on infants and the very young.

- Forbid sharing of clothing among children, especially hats.

Itching may continue after the actual lice war is won. Usually, this is because of certain chemical residue left behind by the departed lice. Failure to recognize this can lead to overuse of lice preparations by frustrated parents. Irritation and itching of the scalp result.

Read well the instructions that come with the preparation.

QUESTION: I am thoroughly confused. I had a medical checkup, and when the nurse took my blood pressure, she did it in both arms. The left arm reading showed pressure of 120 over 81. In the right arm it was 148 over 96. A repeat showed the same variance. I wonder why the difference. - E.F.

ANSWER: A slight variance, say of five to 10 points, in pressure readings between arms is not unusual. Your variation is beyond the normal expectation. It's hard to say for sure what it means. It can be from a narrowing of one of the main arteries leading to the arm.

The next step is for your doctor to determine. Such a variance may be insignificant if it's causing you no symptoms. If it does represent a fatty buildup in such an artery, you will have to begin doing things we all must do to keep arteries clear - watching cholesterol and other fats, losing weight and quitting smoking, if you smoke. I am mailing you the blood pressure report. Other readers may order by writing: Dr. Donohue/No.4, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

QUESTION: After a 52-year marriage, my wife is asking for a divorce. She says it's because I can no longer fulfill sexual needs. What should I do? - Mr. S.

ANSWER: Don't throw up your hands in despair just yet. We have ways to treat impotence.

First off, you have to find out what's causing it. Most of the time, it's a physical problem, not a psychological one. There are definite and pretty conclusive ways to determine whether the cause of impotence is physical or not.

Once the problem is identified, a number of options lie open - injections, vacuum devices, implants, etc. Don't discard a 52-year marriage for something that may have a suitable answer.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.