This is a riddle, but it is not a joke:

What's 19 feet tall, weighs around 400 pounds and is mad at Ronald Reagan?

If you guessed a politically motivated giraffe, you guessed wrong.

The correct answer is a stack of mail. A big stack of mail.

We estimate that the stack has about 50,000 letters. That is the equivalent of one letter for every eight government employes in the Washington area.

The letters are in response to a poll we ran in this space April 17.

The poll asked readers what they think about administration plans to change the federal retirement system, and to revamp rules covering pay raises, promotions and firings. We ran it because Reagan administration officials had been saying they had very little reaction from federal workers, and that much of the reaction they had was favorable to their proposed changes.

They can't say that any more!

Most of the mail came from the Washington area. But several thousand letters--we are still counting--have come from out of town. Postmarks represent about half the 50 states. We have also heard from American civil servants stationed in Japan, Korea, England, Germany, Italy, and in several Latin American nations.

Many people included notes and letters with their ballots. They spell out how many people feel about what is happening to the company they work for--the U.S. government.

We've heard from employes in just about every federal agency, including the CIA, the Tennessee Valley Authority and the National Security Agency.

The vast majority of the voters say they believe that it is unfair for the government to change its retirement rules (raising the retirement age to 65) for current employes. Most people say that if they had it to do over again, they would not come to work for the U.S. government.

One Justice Department voter said he would "disinherit my son and daughter if they took a federal job."

A Foreign Service officer stationed in Belgium said, "The retirement reforms are chilling" and morale among his colleagues is "lower than a whale's belly."

Closer to home (Arlington) one man wrote: "As a lifelong Republican, veteran and federal government worker, I am in support of President Reagan's cost-cutting program. However, should we make . . . the government worker pay so dearly, all at once?

" . . . While I could live with some of the cutbacks, I wonder what we did to deserve such punishment in so short of a time? I have seen the work force greatly increase in quality since the 1960s when I joined the government . . . . The government offered me good benefits, security and a good retirement system for less pay than a private sector job. But then the trade-off was worth it.

"Now, with security, benefits and the retirement system going downhill fast, a move to the private sector is logical for me." If the proposals go through, he predicts that the result will be "an inferior, old, work force with poor motivation."

This is what the administration is proposing:

* The creation of a uniform performance rating system. Employes would not get within-grade pay raises (now virtually automatic based on time-in-grade) unless they met seniority requirements and got a good performance rating just before the in-grade raise was due. Employes could no longer appeal denial of an in-grade raise.

* To devalue seniority when deciding who gets laid off. The proposed new rules would select employes to be RIFed from among groups of workers with the lowest performance ratings. An employes' seniority would be a factor only within his performance rating group. The new rules would also permit agencies to narrow "competitive areas" within which RIFs would take place.

* Narrowing the already limited issues that federal unions can bargain over with agency management.

All of the above changes could be made administratively. They were outlined in the Federal Register in March. The comment period on those proposed regulations is up May 31. The changes could be put into effect any time after that, unless Congress specifically prohibits them.

Legislation would be required to change the retirement rules. President Reagan's plan--which has not yet been sent to Congress--would raise the age for retirement on full benefits to 65, and increase employe payments to the civil service retirement fund (now 7 percent of salary) to 11 percent.