Drug companies are preparing to enter what could be a $1 billion a year business aimed at helping people improve their memories.

The focus is on increasing levels of the neurotransmitter acetylcholine, which falls to abnormally low levels in Alzheimer's patients and may be implicated in other memory disorders as well.

Among the avenues under investigation are drugs and foods including:

*A lecithin-enriched chicken noodle soup designed to increase production of acetylcholine. Now under testing by the Thomas J. Lipton Co. (in consultation with Dr. Richard Wurtman, director of the Massachusetts Institute of Technology Clinical Research Center), the soup theoretically may help people with the memory-impairing Alzheimer's disease or those with tardive dyskinesia -- a debilitating condition that impairs voluntary movement.

*Phosphostigmine, a drug that decreases the breakdown of acetylcholine in the brain and body, and could help boost memory.

*Serotonin, formed from the amino acid tryptophan. In sea snails, serotonin is necessary for a kind of memory/learning known as sensitization, in which the animal learns to respond to something threatening. Increasing serotonin may help some nerve cells to function better.

*Noostropics, a new class of drug that restores acetylcholine levels. One drug, Piracetam, developed by the Belgian company UCB, is one of the first of this class of drugs and is now sold in Europe.