An expert on horse racing can explain to you why trainers don't even bother to enter fillies in the Kentucky Derby. Any competent handicapper can explain why it is foolish to bet on a filly in a race.

In retrospect, we can all explain why a filly did win the 1980 Derby: She got to the finish line before any of the other horses did, that's why.

Those who attended paid a rather handsome price to participate in these events. The winners ate heartily, the losers drank enough to dull the pain, and a total of $10,122,067 was wagered during the day.

Inasmuch as the District of Columbia will soon be voting on a proposition to legalize dog racing and jai alai here "to raise revenue for the city's treasury and thereby minimize the need for tax increases," it might be useful to take a look at how much revenue was raised for the Commonwealth of Kentucky, and at what cost.

Proponents of legalized gambling here would have us believe that gambling would construct a pipeline from the pockets of bettors straight into the District of Columbia's treasury. They neglect to mention that their pipeline will take a few detours, the final one through their own pockets.

The latest information available to me indicates that Kentucky permits thoroughbred tracks to take 15 percent "plus breakage" (at least an additional 2 percent) out of each betting pool. Of this amount, the commonwealth gets 4.75 percent.

Several states have recently raised the "cut" from betting pools to help the tracks meet rising costs and/or put more money into state treasuries. However my reference books do not cover recent changes, so I'll assume that Kentucky's cut of the mutuel handle remains 15-plus-2 percent -- and if it does, it is one of the nation's lowest tax rates on wagering.

If 17 percent was deducted from the $10,122,067 wagered on Derby Day, bettors lost $1,720,751. Of this amount, only $480,798 went to the commonwealth. The other $1,239,953 was paid out for various purposes, including profits for Churchill Downs. So one could say that the pipeline that runs between a bettor's pocket and a state treasury has more leaks than a knitted bathtub. Of the money lost in betting, 27.94 percent went to the treasury and 72.06 percent went elsewhere.

When one considers the other expenses a bettor encounters, the percentage that emerges from the state's end of the pipeline "to reduce taxes" is far less than 28 percent of what vanishes from the bettor's pocket.

For example, the crowd that parted with $1,720,751 in betting losses for the day shelled out about $2 million or $3 million more for admission tickets, programs, parking fees and transportation to and from Churchill Downs.

I do not count the cost of food and "beverages" (such as 100,000 or more mint juleps at $3 each) because people eat and drink whether they're raising revenues for their state treasuries or they're not.

I think a system of taxation that extracts $7 or $8 from the pockets of taxpayers to produce $1 worth of tax revenue is not a model of productivity. If we're going to vote for legalized gambling, let's do it honestly and admit that our basic purpose is profit -- either from betting or profit from providing the facilities for betting. Noble talk about generating painless revenues so that taxes won't have to be raised insults our intelligence. POSTSCRIPT

In case you're interested, $132,913 was bet in the "win pool" on the winner, Genuine Risk. Our news services didn't tell us the total amount in the win pool, but one can calculate it from the payoff odds (13.30 to 1), which are known. About $2,224,000 must have been bet in the win pool.

In other words, those who bought "win" tickets on the other 12 horses lost more than $2 million.

That's just in the win pool. For one race.

My friends tell me that going to the track is lots of fun, but I really don't think I can afford that much amusement. I wonder what's in second place on their entertainment recommendations -- lighting cigars with $10 bills? p PICKING ONE WINNER IS DIFFICULT ENOUGH

Bob Orben comments, "I wish we didn't have to vote for a president and a vice president at the same time.I've never been lucky with exactas." TWAS EVER THUS

Don Epperson of the Texas Tourist Council wonders why it is that the first one to see a traffic light turn green is usually the driver of the second car back. ADD SIGNS

Sign on Interstate 465 reported by hoosier sage Herm Albright in the Perry Township (Ind.) Weekly:

"All motorists who want to save gas, raise your right foot."