The staff nurses' strike at the Washington Hospital Center entered its second day yesterday with morale apparantly high both on the picket lines and inside the Washington area's largest private hospital.

The hospital's emergency room, which had initially been opened only to those ambulances carrying the most critically injured, was restored to full use yesterday morning after hospital officials said they had determined they had the staffing and empty beds to accommodate an increase in emergency admissions.

Yesterday the hospital center had 331 patients for a 38.6 percent occupancy rate. The 911-bed facility, which is almost twice as large as any other private hospital in the area, usually has an occupancy rate of about 78 percent.

Thus far, an average of about 60 percent of the staff nurses have failed to report for work as scheduled. The hospital has been functioning with the staff nurses who have reported to work, plus some temporary nurses and about 100 nursing supervisors not represented by the Washington Hospital Center Nurses' Association, an affiliate of the District of Columbia Nurses' Association.

There are about 425 staff nurses at the hospital and union officials predicted that 300 of them would honor the picket lines. So far they have been within about 10 percent of their prediction.

Officials of both the hospital and the union agree that the union's true test of strength will come tomorrow when the holiday weekend is over and more nurses are scheduled to work. The hospital has said all along that it will assess its position in light of staffing patterns tomorrow.

One question hospital administrators will have to face at that time is what to do about nonnursing support personnel needed to run a 911-bed hospital, and who continue to work during the strike but are not needed to run a hospital filling less than 400 beds.

The nurses, too, will learn tomorrow how effective their picket lines are in stopping deliveries of supplies to the hospital. Union officials say they have been assured that members of the Teamsters Union will not cross the nurses' picket lines, but at the same time hospital officials say they have been assured by their suppliers that needed goods and equipment will reach the hospital.

Sources on both sides say they expect negotiations, being held under the auspices of the Federal Mediation and Conciliation Service, to resume Tuesday. The sessions were recessed Saturday morning after an all-night marathon failed to produce a contract.

Money is not a major issue in the dispute between the nurses and their employer. Rather, the more basic question of who is going to control nursing at the hospital - the union or the hospital administration - is at stake.

The questions preventing a settlement include the setting of work schedules, how much vacation and sick leave a nurse may accrue and who will decide when and how much maternity leave and educational leave a nurse may take.

The dispute comes at a time when some nurses are experiencing a newly developing sense of professionalism involving a vision of themselves as equal partners with physicians in health care rather than as minions who stand by waiting for orders.