About two years ago, Emily Myatt wrote to Rep. Stan Parris (R-Va.) about the need to protect the bald eagles in the Mason Neck Wildlife Refuge along the Potomac River.

"Ever since then, about once a week, I get a letter from him," Myatt said laughingly.

While the letters probably have not been that frequent, Parris offers no apology for filling Myatt's mailbox with frequent, personally addressed letters about the Fairfax County wildlife refuge, environmental issues and actions that might affect her.

The reason: Parris runs a computerized office -- some say one of the best on Capitol Hill -- producing 4,000 to 5,000 letters a week. It is run by a Texas Instruments computer linked to seven terminals and a list of 200,000 names of persons such as Myatt who had written to the 8th District congressman.

Each time a constituent writes to Parris about an issue, Parris aides enter the letter writer's name into the computer under any of 1,640 issues. As Parris aide Dick Leggitt put it: "Once you write Parris, you're in our system forever . . . . "

Once the name is coded into the Parris computer, little more than typing a sample response is required to prepare a letter that can be quickly addressed and sent to anyone who has written to the congressman on the subject. And with Parris, that happens frequently.

While Parris aides brag about the operation, Democrat Richard L. Saslaw, Parris's challenger in the fall election, sees the computer as giving the incumbent "an unfair advantage." Saslaw calls the system "a multimillion-dollar direct-mail operation," citing the staff time and the free mailing privileges Parris has.

"A postal worker told me he had fallen arches from carrying this guy's Parris' stuff around," says Saslaw. Parris rejects Saslaw's charges and says the computer simply enables him to run a more efficient office. "This is just a better way to communicate with constituents," said Parris.

The operation costs taxpayers about $2,500 monthly, about half for leasing the equipment and half for access to data banks, he says.

The Parris staff says that the computer operation has been a huge success: a recent poll showed that voters who liked Parris often say it is because he keeps them informed.

"When we came in in 1980, we won by only 1,100 votes" over Democratic representative Herbert E. Harris, said Leggitt. "Had Harris' people taken advantage of the technology available, we might not have been able to pull the upset that we did."

Parris is not alone in grasping the political impact of a computerized congressional operation. Only two or three congressmen have offices that are not hooked up to a computer, although many have far less sophisticated operations.

"He Parris is just using the letters to embellish himself," said Saslaw, who charged that Parris has distorted or inflated his role in legislative actions in the letters.

Parris aides deny Saslaw's charge. "The letters are often a correspondence version of how a bill becomes law," said Leggitt.

When Parris wants to get the word out that he has done something or that there has been action on a particular issue, a letter is sent out to each person coded for that topic. Currently his biggest mailings are to the 25,000 persons who have written about the District-run Lorton prison in southern Fairfax County. Parris has been outspoken on the issue, arguing that the prison should be moved out of his district.

More typical are the 3,500 persons who wrote urging funding for the Mason Neck refuge. They received an initial letter from Parris saying he agreed with them, and each time a key action in the House or Senate occurred, another letter explaining what had happened and what his role had been.