Lawrence Horowitz must be the most underpaid staff aide in the Senate. Since 1984, Horowitz, administrative assistant to Sen. Edward M. Kennedy (D-Mass.), has been paid an annual salary of $3,000, just enough to qualify him for Senate security clearance and coverage under the Senate's health insurance plan. It is less than the federal minimum wage.

But Horowitz is not complaining. While he has continued to work as Kennedy's top aide, Horowitz, who is also a physician, has been engaged for the past two years in some lucrative outside ventures, acting as an adviser on health matters to several corporations and as a consultant to a firm that makes investments in medical- and health-related businesses.

The apparently unusual arrangement was worked out with Kennedy and cleared in advance by the Senate Select Committee on Ethics, which ruled that, with the understandings and restrictions agreed to between Kennedy and Horowitz, this outside consulting work did not violate the conflict-of-interest provisions of the Senate Code of Official Conduct.

The Horowitz case is one of thousands that have been considered during the past few years by the Ethics Committee, the guardian of propriety and the appearance of propriety for the Senate.

Created in 1977, the committee has jurisdiction over Senate rules on such matters as outside employment, gifts, financial disclosure, foreign travel and the franking privilege.

According to Bonnie S. Parker, the committee staff director, the committee has issued about 3,300 written opinions on these and related questions to senators and staff aides seeking advice on what is and is not permissible.

In addition, many more informal opinions have been provided over the telephone by Ethics Committee staff attorneys.

Last year, the committee published for the first time a compilation of its "interpretive rulings" -- advisory opinions in 394 cases with broad enough application to be used as general guides.

Several of these deal with outside employment and potential conflicts of interest, the bulk of them stemming from inquiries made in the first years after creation of the committee and passage of the Ethics in Government Act of 1978.

May a Senate employe accept an unpaid "of counsel" relationship with a law firm? No, the committee ruled, that would violate a prohibition against allowing an employe's name to be associated with an outside firm.

May a legislative aide for agricultural affairs own a company that bears his name and operate a dairy business from which he receives rental and dividend income?

Yes, the committee said, because the prohibition against association with an outside firm covers the "major professions" such as law, medicine and engineering, but not farming.

May a Senate aide sell real estate on weekends under a broker's license held by a real estate company? No, the committee said, because that would amount to affiliation with the real estate company.

Much depends on the circumstances in each case. In a 1979 interpretative ruling, for example, the committee reminded Senate employes that a committee staff aide who resigns and joins a registered lobbying firm must refrain from lobbying members or staff of his former committee for a year after leaving the Senate.

But in other cases, the committee has ruled that senators may hire on a part-time basis persons who work for registered lobbying firms but do not personally lobby Congress.

The key in this and several other cases is how the individual promises to behave.

For all of its elaborate, footnoted structure, the Senate ethics system still depends largely on trust and individual integrity.

There is no requirement that anyone seek an advisory opinion from the Ethics Committee.

Committee approval of certain arrangements amounts to "official clearance," Parker said, but there is no monitoring to determine if the conditions of approval are being met.

In cases in which the committee has advised against certain practices or arrangements, no attempt is made to determine whether the advice is being followed.

The burden of enforcement rests with each of the 100 senators and the supervisors of their offices and with committee aides.

In the Horowitz case, the burden rests with Kennedy. He wrote to the Ethics Committee in 1984 detailing the proposed consulting arrangement, from which he said Horowitz would derive "substantial outside income."

He pledged that Horowitz would consult only on health benefit plans of his client corporations, the health and fitness programs of their employes and particular medical problems of individual employes, that he would not discuss other issues with the corporation and that he would not be used as a means of communications between them and Kennedy.

Horowitz, Kennedy told the committee, would not be affiliated with the corporations or allow his name to be used by them, and would not discuss with Kennedy any issues involved in the consulting contracts.

Under these conditions, the arrangement was approved by Sens. Ted Stevens (R-Alaska) and Howell Heflin (D-Ala.), then the chairman and ranking minority member of the Ethics Committee.

It was not reviewed by the full committee; according to Parker, this is the usual practice when the two senior members agree and the issues are not considered particularly difficult.

Horowitz said that he is currently a health consultant to five companies. He declined to disclose the names of his clients or his outside income, but said it exceeded the normal salary for a Senate administrative assistant, which ranges up to -- and in a few cases past -- $70,000 a year.

Horowitz said he remains with Kennedy out of "personal loyalty" and denied that his association with the Massachusetts senator, which stretches back to the early 1970s, benefits his outside work as a consultant.

"There is no possible conflict," he said. "I could go out tomorrow and get 800 clients."