Allyn Cox, 86, an artist whose work includes some of the major murals and friezes in the U.S. Capitol, died at the Washington Hospital Center on Sept. 26 following a stroke.

In 1953, Mr. Cox was commissioned to complete the Rotunda Frieze just under the dome of the Capitol. The paintings were begun in the 19th century by Constantino Brumidi, the Italian artist who spent much of his professional life working on the Capitol. Brumidi died before the work could be completed.

This circumstance provided Mr. Cox with an ambition that began in his boyhood: to complete what Brumidi had started. "When I was very young," he said in an interview shortly before his retirement last April, "my parents brought me here and showed me an empty space in the frieze under the Rotunda dome. After that, I used to dream and dream of painting it one day."

When the call came, he mounted a scaffold and painted in the 32-foot space with figures depicting the American Civil War, the Spanish-American War and the birth of the age of aviation. All of it was faithfully executed in the style of his Italian predecessor.

In 1969, Mr. Cox again was called to the Capitol. This time the job was to clean and restore the Brumidi mural called "The Apotheosis of Washington," which is at the very top of the Rotunda.

Before that task was over, Congress authorized the decoration of various walls and ceilings on the House side of the building. Brumidi himself had done a number of murals in the corresponding corridors in the Senate wing.

The actual work began in 1973 under the auspices of the U.S. Capitol Historical Society and the Daughters of the American Revolution. Mr. Cox and two assistants, Cliff Young and John Roach, did the ceiling of the Capitol Hall with scenes showing the development of Congress. In the Great Experiment Hall, they covered the ceiling with paintings of the growth of American democracy.

The work was dedicated earlier this month and on Sept. 21 a ceremony in Mr. Cox's honor was held in the Statuary Hall with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Senate Majority Leader Howard Baker (R-Tenn.) in attendance.

Inevitably, perhaps, Mr. Cox's work has been compared to that of Michelangelo. When the validity of this was put to him in an interview, Mr. Cox replied with puckish good humor. "Well," he said, "the ceiling of this corridor is longer than the one in the Sistine Chapel."

Mr. Cox was born in New York City. He studied at home -- his father was a painter -- at the National Academy School and at the Art Students League. He served in the Red Cross in Europe during World War I, then came home to begin his career.

Having already decided that murals would be his milieu, his first major effort was to paint a room in his parent's house in New York. This brought a number of commissions from private persons, including Vincent Astor and others among the very rich. With his father, he also did some murals in the Wisconsin State Capitol in Madison.

With the onset of the Great Depression and the consequent disappearance of large private commissions, Mr. Cox turned to decorating public buildings. Those on which he worked included the law school of the University of Virginia. He also worked on the George Washington Masonic Memorial and the Knights Templar Chapel in Alexandria.

His wife, Lillian, died in 1968, and Mr. Cox had lived in Washington. Survivors include a sister, Carolyn Lansing of Apache Junction, Ariz.