Last June, Roni Robins, a 31-year-old assistant planner in the Fairfax County Office of Comprehensive Planning, was directing the county's review of a $1 billion development that required several major rezonings. Relying largely upon her advice, the county agreed to the developer's requests and approved the mammoth project known as Kingstowne.

Three months later, Robins again was immersed in the Kingstowne project, a housing and commercial development south of Alexandria. Only this time, she was managing the project for the developer's subcontractor -- KMWM Corp., the principal engineering and site planning firm working on the project.

Robins said KMWM did not approach her until after the Kingstowne rezonings were approved and said she saw nothing improper in her move to private industry, a transition that several dozen other experienced Fairfax County planners and engineers have made since the early 1980s.

Most are signing on with developers. Others are being lured by engineering and environmental firms that do consulting work for developers. Most are hired at salaries at least 25 percent above their county pay, according to Supervisor Audrey Moore (D-Annandale).

Fairfax officials say the exodus has left a vacuum of expertise in the county's planning bureaucracy and has raised concerns about potential conflicts of interest. Unlike the District of Columbia and Montgomery and Prince George's counties, Fairfax County has no restrictions on employes' dealings with the government once they leave it, a fact that troubles a number of county officials.

"I cringe when I think of it," Carl L. Sell Jr., a member of the Fairfax County Planning Commission, said of Robins' departure. "I don't question her integrity, but I was disappointed she would go to work for the other side. I just don't think the public perception of that is good for the county."

"It's like having the fox in the chicken coop," Sell said, alluding to the roles being assumed by Robins and other former Fairfax government employes. "They certainly have valuable . . . information from working for the county. I don't know whether they would try to abuse their former positions or not, but the opportunity is there. I've got some real concerns about that."

To Robins, the new job climaxed a natural career progression from government to the private sector, creating an opportunity to use her expertise and to make more than the $32,393 a year that Fairfax County paid her. She declined to disclose her current salary and said her past government role does not give her new employer any unfair advantage.

"I think it's very difficult if not totally impossible to cut corners because there are so many people reviewing the plan" in county government, Robins said. "Knowing the process and the people to go to saves time. Being able to inform the developer of the county perspective also saves time. But I don't think it changes the end result."

Although county officials contend that the prospect of higher salaries is luring away employes, several who have left say their pay raises have been more modest than the 25 percent cited by Moore.

They mention a variety of additional factors. Chief among them are limited opportunities for advancement in government and an emphasis on dealing with day-to-day crises -- many political in nature -- rather than on long-term creative planning.

Whatever the reasons, Fairfax has been unable to hold even some of its top officials.

Their departures have decimated the ranks of the Office of Comprehensive Planning, where recommendations are made on planning and zoning that can mean millions of dollars to developers.

In January, Sidney R. Steele, director of the office, resigned to become director of development for the Webb-Sequoia Co. Theodore J. Wessel, Steele's predecessor, left in October 1984 and is now vice president at A.J. Dwoskin Co., a real estate and development firm. Philip G. Yates, Fairfax's zoning administrator, stepped down 16 months ago to join the planning division of Dewberry & Davis, a Fairfax planning consultant.

Other area governments, particularly in the District and Montgomery and Prince George's counties, also have suffered the loss of employes to the private sector. Officials there say they are concerned by departures, but note that they have laws regulating their employes' dealings with the government once they leave.

Fairfax officials long have resisted efforts to adopt such a law.

"I don't see how you can do it. Where do you draw the line?" said Fairfax County Executive J. Hamilton Lambert. "I've looked at it in the past, and it's fraught with problems. There might be some way it could be done, but I'm not aware of it."

Neither Arlington nor Alexandria has such a law, and Fairfax County Attorney David T. Stitt said the localities would have to seek special state legislation authorizing them to enact such a provision. There is a Virginia law that restricts post-government jobs by procurement officers, but no similar law dealing with planning officials.

More and more jurisdictions around the country are seeing the revolving door between government and private industry as a problem and are confronting it by adopting tough ethical safeguards.

"There's a growing awareness of it," said Alan Beals, executive director of the National League of Cities. "From an ethical point of view, it makes good sense to have some time elapse before an employe comes back for a company in front of the government agency he worked for. There needs to be an arm's-length relationship."

In Montgomery, a former county employe is prohibited from assisting or representing a firm "in a case, contract or other specific matter if that matter is one in which he significantly participated" while with the Montgomery government, said David Frankel, an assistant county attorney.

The Montgomery ethics law also precludes employes from using their government positions to negotiate for jobs in private industry.

Similar provisions are on the books in Prince George's County. County Attorney Thomas P. Smith said they are designed "to ensure that county officials and other employes give the county their undivided loyalty while in the government.

"It's to make sure that the county employe is not providing favoritism to a potential private sector employer, trying to feather his nest" while working for Prince George's, Smith said. "And it's to make sure that the employe does not take advantage of the county when that employe goes to the other side."

The District is covered by an ethics code that includes a one-year prohibition against senior employes representing any firm in dealings with the department or agency for which they worked.

Some Fairfax officials favor the adoption of a similar law to govern the conduct of their present and former employes.

Proponents of such a regulation, including Supervisors Moore and James M. Scott (D-Providence) and a number of Planning Commission members, contend that the county easily could pattern an ethics law after others in place in nearby jurisdictions.

They argue that, at a minimum, Fairfax should prohibit its former employes from representing a company in any way before the county on a project with which they were involved while in the government. To do otherwise, they say, not only creates the potential for unethical activity or unintentional abuses, but erodes public confidence in the county government.

"It just looks awful," Moore said of the revolving door between the Fairfax County government and the development community. She said that the free and unrestricted flow of county employes to private industry gives developers "a special knowledge and a special access they should not have."

Moore cited Yates, the former zoning administrator, as an example. Now at Dewberry & Davis, Yates left the government in January 1985 after nearly 20 years with the government. Moore noted that Yates, as zoning administrator, helped write and interpret the zoning laws during the county's period of rapid growth.

"Now he can advise developers about how to get around those rules," Moore said, emphasizing that she was concerned about the potential for circumventing regulations and was not accusing Yates of any impropriety. "Who better would know about it than the man who wrote the rules to begin with?"

Yates declined to be interviewed for this article.

"There should be a cooling-off period. There's no question in my mind about that," said Sell, the planning commission member who represents Fairfax's Lee District. Sell said Fairfax should prohibit its former employes from contacting county officials on projects they once handled for at least a year after they have left their county jobs.

Fairfax County Planning Commission member Ronald W. Koch agreed on the need for such a prohibition. He said the flight of county employes to the private sector has raised concerns about whether certain developers will receive favored treatment. Moreover, he said, residents are legitimately concerned that some county employes may not have the taxpayers' best interests in mind when drafting planning and zoning guidelines, approving building permits and seeing to it that developers do not cut corners in construction.

"A lot of people have the perception that Fairfax staff members may have gone easy on a particular developer" with the thought of landing a job with that developer, Koch said. "These people are representing the citizens of Fairfax County, and they should be giving their 100 percent loyalty to the county and to the taxpayers who are paying them.

"Unfortunately," Koch added, "as long as county staff continue to accept employment with companies with whom they had been dealing on behalf of the county, the perception will be that the remaining staff are in training for the private sector."

Among the cases recalled by officials is that of Jeffrey Saxe. A onetime senior planner for Fairfax, Saxe played a key role in the county's review of the Hazel-Peterson Co.'s Fair Lakes project, a huge residential and commercial development near the Fair Oaks Mall.

Less than a year after the Fair Lakes development received county approval, Saxe accepted a job with Hazel-Peterson, the firm headed by John T. (Til) Hazel, one of Northern Virginia's most prominent developers.

During the county's review of Fair Lakes, Saxe said he "never had one discussion" with Hazel-Peterson about his job and never had "one thought that was what I wanted to do."

Saxe, who earned $35,905 a year with the county, said he changed jobs because of the opportunity for greater flexibility and a higher salary. He said he has not tried to use whatever influence he may have with county officials to assist his new employer.

"As long as one is not using private information not available to the development community in general, there's no conflict," Saxe said.

Claire M. Guidas worked as a Fairfax transportation planner from November 1982 until April 1985. One of her chief responsibilities was helping draft a transportation plan for Centreville, a rural and largely undeveloped section of the county. The question of how it should be zoned sparked a fiery debate between residents and developers.

While the Centreville issue was still under review, Guidas, who earned $29,663 a year with the county, accepted a job with Dewberry & Davis, the Fairfax engineering and architectural firm under contract with a number of developers, including some of those involved in the Centreville dispute.

"I feel comfortable with it," Guidas said in an interview. "There is no conflict there."

Other former county planners contacted last week shared that assessment.

"Unless you're using some sort of friendly relationship [with a county employe] or you're working on the same case you worked on [as a Fairfax employe], there's no problem," said Carlos M. Montenegro, a senior county planner until last September, when he accepted a job as project manager for the Centennial Development Corp.

"I do the same thing I used to do [for the county] except that now I'm on the other side," Montenegro said when asked to describe his position with Centennial.

Richard M. Reid, another former county planner who is a project coordinator for Paciulli-Simmons & Associates, a surveying and engineering firm, said that hiring county employes gives developers a definite advantage.

"They're taking on people who know the ins and outs, who to talk to and how things work" in government, he said.

Reid said that he feels comfortable "picking up the phone and calling somebody I know on the [planning] commission or the board [of supervisors] to arrange a meeting" and that he does not believe he is abusing the system.

"I don't feel I get any special treatment because I used to be a county employe," he said.

Denton U. Kent, deputy county executive for planning and development in Fairfax, said that the hiring away of his staff "is an advantage to a developer in that there may be some shortcuts suggested [by] someone who knows how the system works." But, Kent added, former employes "are not granted favors. I don't think there's necessarily that old-school tie."

At Dewberry & Davis, where several former Fairfax planners are employed, Sidney O. Dewberry dismissed suggestions that hiring those officials has given his firm an advantage over his competitors. Dewberry also said he does not use those employes to seek an upper hand in negotiations with the county.

"We've been very careful about that," he said. "We try to avoid situations that would be embarrassing to the employe, the county or ourselves."

Still, many Fairfax officials suspect that the exodus to the private sector is undermining the county's planning and zoning process and creating the potential for conflicts of interest.

"There's a much greater potential for a developer to cut corners on the overall quality of a development" by relying on the advice of a former county planner familiar with Fairfax's procedures, said Sell, the planning commission member. "I just don't think the perception of that is good for the county."