Information on everything from animal control to zoning laws and wood-burning stoves to building permits is now only a telephone call away in Fairfax County, thanks to a new tape-recorded information service available from the county 24 hours a day, seven days a week.

The comprehensive system, unveiled at a Board of Supervisors meeting earlier this month, is the first in the Washington area and one of the first in the nation, said county spokeswoman Jean Van Devanter.

Only residents with Touch-Tone telephones can receive information via the system, which covers more than 250 specific topics in 19 major categories.

All they have to do is to dial 691-INFO (691-4636) and then a three-digit code assigned to each topic. In the health and medical service category, for example, callers can choose from 11 different tapes: immunization, venereal disease, Medicare, Medicaid, hospital care, home care, mental health, mental retardation, alcohol abuse, drug abuse and air pollution.

Other major categories include consumer and legal services, employment and training, housing and construction, motor vehicles, parks, police and courts, public works, real estate, social services and voter registration.

A complete list of the tapes available is provided on tape number 338.

Board Chairman John F. Herrity called the new system "tremendous." He stressed that the system is "not a replacement of human citizens, but a supplement to them."

Residents should call county offices if they have a question or concern that they feel is beyond the scope of the tape-recorded messages, Herrity said.

Van Devanter said it will cost $9,600 a year to rent equipment for the "Facts Information Telephone Line" from Apex Technology in Falls Church. The equipment will be leased at least until the county completes a cost-benefit analysis on purchasing it, she said. The county also plans to spend $32,000 to mail detailed brochures explaining the system and listing the codes to all households early next year.

Van Devanter said the message system received 500 calls on Dec. 10, the night that it was announced by the board, and 388 calls the following night. "People seem to like it," she said.

The county now averages close to 200 calls per day, said spokeswoman Ann Anderson. The most popular request by far is for information on taxes, she said.

In addition to enabling callers to receive information at any time of day, the system will save money, Van Devanter said.

A study by the Internal Revenue Service of a similar message system showed that the cost of responding to an average inquiry under an automated system averaged 82 cents, compared with $2.09 per inquiry for an employe to respond. The system thus should pay for itself within a year, Van Devanter said.

She said the county kept costs down by employing work-study student Cheryl Dolan from George Mason University to write many of the scripts used in the system.

The county also used staff employes to record the messages, instead of contracting for the narrations. This, she estimates, saved the county about $5,000. Police public information officer Warren Carmichael, economic development public affairs director Theda Parrish, Dolan and Van Devanter read the scripts.

But the new system still has a few glitches that need to be worked out. A call to one of the county's most popular tapes, on calculating taxes, demonstrated that even the government has a hard time computing these elusive figures.

Tape 433 contains the following 35-second message: "The tax rate is expressed in dollars and cents per $100 assessed value, and the actual tax is computed by multiplying assessed value by this rate. The real estate tax rate for calendar year 1984 is $1.46 per $100 assessed value, which means that the annual tax on a property assessed at $100,000 will be 100 times $1.46, or $1,460. Thank you."

Actually, 100 times $1.46 does not equal $1,460, as the tape says. The correct multiplier should be 1,000.

It just goes to show, said embarrassed county officials, that even with state-of-the-art technology, "we're only human."