The northern half of University Hall was practically deserted yesterday. One lone secretary, after finishing work at eleven p.m. the night before, spent the day answering telephone calls that tricked in from disappointed parents.

The thick steel first floor doors inscribed "Admissions Committee" were open during the day for the first time in two months. Inside, windows were raised, airing out the room. There was little indication that the committee had been working with its staff of secretaries right up to the midnight deadline to get more than 3,600 acceptance and rejection slips into the mail.

The pattern was not new. Dean Bender and the committee had been up until midnight almost every night for two months, sorting through a record number of applications for the Class of 1958. The job involved holding interviews, scrutinizing transcripts, reading recommendations, and withstanding outside pressure.

Over the week-end, the University secured an outside room for Bender. Closeted there, the Dean of Admissions began dictating letters. "Dear Sir: The Committee on Admissions has decided to accept your son, but has grave doubts . . ." Or: "After reviewing your son's secondary school record, we feel that we cannot accept . . ." Or: "Our conference with you last week was most satisfactory . . . Please find enclosed your certificate of admission . . ."

All letters went into the mail Monday at 11:30 p.m. Airmail letters to west of the Mississippi will not arrive until today, but by 10 yesterday morning parents and sons in the Cambridge area had received theirs. Some of the rejects began to call the northern end of University Hall.

Behind a desk littered with carbon copies of the week-end's dictation, Dean Bender's secretary answered calls. "Yes," she said, "I sympathize with you, but . . ." Then she asked the parent to call back in a day, when one of the admissions men would be glad to speak with her. Yesterday, however, after two months of overtime work, the committee took its first day off.

Much telephone work lies ahead, but yesterday Dean Bender was sleeping.