Washington is facing its second strike in less than a month by health care professionals: Registered Nurses at Washington Hospital Center have served notice that will walk out at 7 a.m. Saturday if they do not have a contract.

Officials of the fledging union that represents the hospital's 500 registered nurses say they expect about 300 of their members to join the strike.

The nurses say the dispute is about working conditions and fringe benefits more than wages.

The situation is complicated by the fact that the hospital last month withdrew recognition of the union as the official bargaining agent of the nurses. It contended that the union did not represent a majority of the 500 RNs.

The nurses have challenged that action in a complaint filed with the National Labor Relations Board, which is investigating the matter.

Yesterday the nurses attempted to deliver a letter to Allen G. Siegel, the attorney who represents the hospital, in which they charged that "nothing but the arrogance of the Washington Hospital Center leadership prevents us from reaching an agreement which is fair to all concerned."

Officials of the hospital have refused during the past week to discuss the possibility of a strike or to say how they intended to staff the hospital if the nurses go out.

Washington Hospital Center has 911 beds and is the largest private hospital in Washington. It has a 73 percent occupancy rate and treats more than 41,000 patients a year in its emergency rooms.

If the nurses do strike, they would not take 100 nursing supervisors with them. These employes, together with the 200 nonsupervisory nurses not expected to strike, would give the institution half its normal contingent of 600 supervisory and nonsupervisory registered nurses available for duty, the nurses predict.

Among the key sticking points in the negotiations, which broke down last December are:

The nurses' demand for maternity leave. The hospital's last offer does not guarantee maternity leave, the nurses contend, but rather leaves the granting of such leave up to the individual nurse's supervisor.

The assigning of shifts. The nurses want seniority and for nurse's preference to play a role in the assigning of work hours; the hospital maintains the right to assign as it sees fit. In addition, the nurses are demanding the right to be assigned to permanent day shifts and the hospital has aid it will not grant such work schedules but will allow permanent assignments to night duty.

Time off for continuing education. The hospital maintains that the granting of educational leave is a discretionary right of a supervisor, and would only allow the granting of one day of leave in the contract's first year and two in the second. The union has demanded five days of educational leave a year.

Performance evaluation. The nurses are demanding that performance evaluations be subject to review and change through the grievance procedure. The hospital would only allow nurses to add a written comment to an evaluation, but would not allow a grievance.

Vacation and sick leave. The nurses have been offered one additional day to be added to a pool of combined vacation and sick days numbering 19 in their first two years of employment, 22 days in the third year, 24 in the fourth through 10th years and 10 days after 10 years. That would give a nurse four weeks of vacation and nine days of sick leave after 10 years. The nurses have made an unspecific demand for an increase in the number of days and the right to accumulate days of sick leave.

The nurses are paid betwwen $12,528 and $14,679 a year. The hospital has offered a starting wage of $12,632 and a raise of "approximately" 5 percent in each of three contract years. The nurses have demanded 8 percent.

The troubles at the hospital follow closely an 11-day strike by physicians at Washington's Group Health Association, and come at a time when hospital an insurance plan administrators are under preat pressure to contain highly inflationary health care costs.

The dispute also is occurring at a time when nurses are demanding greated respect and independence as professionals - demanding to be treated as equal members of a health care "team" - rather than as the more traditional serfs in white, doing the bidding of and menial work for physicians.