Leslie C. Arends, 89, an Illinois Republican who served in Congress for 40 years and who was his party's whip from 1943 until retiring from the House of Representatives in 1975, died of a heart ailment yesterday at a hospital in Naples, Fla. He had a vacation home in Naples.

During his years in the House, he rose to a senior position on the Armed Services Committee and was a leading member of the conservative wing of the Republican Party in Congress. Although the GOP controlled the House for only four of the years in which Mr. Arends served in it, it was as whip, or deputy party leader, that he made his mark.

The job of a whip is to act as a channel of communications between the party leadership and the members. He must try to persuade members to follow the leadership and see that the rank-and-file are present for important votes. He must keep them informed about the legislative calendar and know where they are during legislative sessions.

The whip also must keep the leadership informed of what the members are thinking. He must know how members feel concerning various issues and what particular problems they might face in their districts, and he must report his findings to the leaders.

Mr. Arends seemed to thrive on these time-consuming tasks, although they garnered him comparatively little credit in his own district. His problems were threefold: holding his own House seat, keeping the trust of House members, and working effectively with other party leaders.

Mr. Arends kept his seat until he chose to retire. His Midwest brand of conservative Republicanism was typical of his time and place. Before World War II, he opposed Lend Lease, the relaxation of the neutrality laws and naval construction projects. After the war, he voted against organized labor and in favor of farm programs.

He surprised many observers with the ease with which he turned back a 1965 challenge by Peter H.B. Frelinghuysen (R-N.J.) for the whip's post. House Republican leader Charles Halleck had just been defeated for reelection by the more moderate Gerald R. Ford (R-Mich.) and Mr. Arends was thought vulnerable. Yet Mr. Arends defeated his challenger 70 to 59, and was heard to say he had nine more votes in reserve.

Many considered his victory over Frelinghuysen to be the natural result of the qualities that made him so successful on Capitol Hill.Mr. Arends exhibited unquestioned honesty, genuine friendliness, true loyalty and good humor. In a career such as his these seemingly modest gifts can be as important as towering leadership or vaulting ambition.

Leslie Cornelius Arends was born Sept. 27, 1895, on a farm near Melvin, Ill., about 100 miles south of Chicago. One of seven children, he was educated in public schools and attended Oberlin College.

"I was brought up right," he once said, "as a Republican."

After serving in the Navy during World War I, he farmed, and had farming interests in Melvin until his death. He also entered banking, becoming president of the Commercial State Bank in Melvin. The first elective office he ever sought was a seat in the House of Representatives, which he won in 1934.

A noted golfer, he was a member of the Burning Tree Club. He also was a Mason, a member of the American Legion and a trustee of Illinois Wesleyan University.

In addition to his vacation home in Naples, he maintained residences in Washington and Melvin.

Survivors include his wife, Betty; a daughter, Letty Arends Eckel of Brookline, Mass., and one grandchild.