To people who are only casually familiar with the Daily Racing Form, the addition of one more symbol to its already imposing mass of data might not seem big news. But for the newspaper's devotees, a change in the Racing Form's past performances is like a change to the U.S. Constitution or the Ten Commandments. There is no such thing as a minor one.

On July 1 the Racing Form added a boldface, capital "L" to its result charts and past performances to signify that a horse had been treated with the medication Lasix. This information is extremely valuable, because Lasix is such an important factor, and many of us have complained that the Form didn't make it available long ago.

"One of the problems," said George Bernet, the paper's assistant editor, "is that so many states have so many different rules about Lasix." And, he said, the lack of uniformity in medication rules across the nation still is causing problems. While the Form can list horses who will be racing on Lasix in the upcoming day's races in Maryland, it cannot do so in some other states, such as Florida and California, where horses getting medication are not listed at the time of entry.

But the Form will show when horses have raced with Lasix in past starts, and that is the crucial information. While the racing program lists the horses racing with Lasix today, bettors often haven't known if a horse was getting the medication for the first time -- the circumstance when Lasix can bring about a sharp improvement. If a horse did all its racing at Laurel and Pimlico, there was no problem, for Maryland officials kept all the pertinent records for their state. But if a horse was shipped from Garden State Park to Laurel, it was hard to find out if he had raced with Lasix in New Jersey, because there was no interstate record-keeping system.

Now there is, and horseplayers should be able to deal with Lasix as a handicapping factor with relatively few problems. Even though debate continues to rage among scientists, veterinarians and industry leaders about the properties of the drug, horseplayers have developed a pretty good sense of the way Lasix affects thoroughbreds. There are no certainties in this game, of course, but a horse getting Lasix is no more of a mystery than a horse getting blinkers for the first time. Here are the basic handicapping guidelines:

Lasix is a relevant factor only when a horse gets it for the first time -- or, at least, when he has started using it recently. There is no difference, from the handicapping standpoint, between a longtime Lasix user and one who doesn't use the drug.

Many horses improve sharply when they get Lasix for the first time, and the ones most apt to do so are those who have been showing early speed and tiring. They may be quitting in the stretch because they suffer from respiratory problems -- the problems that the diuretic may alleviate. Plodding stretch-runners are less likely to be helped by Lasix.

A horse is an especially good candidate to improve with Lasix if he showed some talent in the past, even though his recent form might be poor. Lasix helps horses by correcting a physical problem that has caused their performances to worsen; it rarely transforms horses who have never shown any ability whatsoever.

If a horse doesn't improve the first time he gets Lasix, he probably won't improve subsequently -- unless, of course, he was entered under impossible conditions when he got his first Lasix. If, for example, a horse who hates mud gets first-time Lasix on a muddy track, he deserves consideration the second time around.

When a horse has started using Lasix and has improved with it, he may be considered a new animal, for all practical purposes. This principal was demonstrated on the day of the Preakness -- a day when Lasix was the main topic of conversation because of Summer Squall's victory with the aid of the drug.

Joe's Lad had been a fair-to-middling Florida campaigner before his most recent race, which he won by 7 1/2 lengths in fast time, and now had come to Pimlico for a stakes race. Handicappers oblivious to the Lasix factor might have dismissed that last race as an aberration and concluded from his overall record that Joe's Lad wasn't good enough to win in stakes company. But, unbeknownst to many in the crowd, Joe's Lad had received Lasix for the first time in his previous start. His strong performance had been no fluke; this was the new Joe's Lad, and he proceeded to blow away his Pimlico rivals.

Most bettors at Pimlico that day were not equipped to evaluate Joe's Lad because the relevant information wasn't available in the Racing Form. Now it will be -- and the handicapping process will be a little bit easier as a result.