H. Jackson Pontius, executive vice president of the 583,000-member, Chicago-based National Association of Realtors, will leave that post at the end of this year. He has held the upperbracket trade association job since 1970.

The early retirement agreement came as a surprise at a final session of the recent spring meeting of NAR board members in Chicago.

Widely known throughout the nation and a frequent to visitor to Washington, Pontius, now 61, is retiring in advance of the mandatory NAR staff retirement age of 65. He will continue to serve NAR as a paid consultant until 1987.

Pontius said yesterday from Chicago that his top staff assistant, William R. Magel, 47, will "likely" be chosen to succeed to the top NAR post before next January. Magel had been executive of the New York Associationjobs in the nation.

During his service with NAR, the membership rise from 91,000 relators to 203,000. Four years ago, associate member status was added, and now there are a total of 583,000 dues-paying realtors and realtor associates.

Pontius also was instrumental in the recent purchase of a 10-story, $8 million headquarters building for NAR near the Tribune Tower in midtown Chicago. NAR also operates its 50-member staff Washington office in its own building at 925 15th St. NW.