Dennis P. Grassi, 53, a forester with the U.S. Forest Service of the Agriculture Department, died Sunday at Alexandria Hospital after a heart attack.

Transferred in April to Ogden, Utah, after serving at the Forest Service offices here for 11 years, Mr. Grassi was in Washington on official business and was visiting friends in Alexandria when he was stricken.

He had been a staff specialist for land acquisition at the Forest Service office here. In Ogden, he was responsible for land purchases and right-of-way acquisitions for the national forests in Utah, southern Idaho, Nevada and western Wyoming.

Born in Bellaire, Ohio, Mr. Grassi was a graduate of Antioch College and earned a bachelor's degree in forest management from Utah University. He served in the Pacific with the Navy in World War II.

He had started work with the Forest Service while still attending college. From 1952 until coming here in 1966, he held forest positions in the Southwest.

He was a member of the Society of American Foresters, the American Society of Real Estate Appraisers and the American Society of Farm Managers and Rural Appraisers. He also was active in The Nature Conservancy, Izaak Walton League and the National Wildlife Federation.

He is survived by his wife, Marie, of the home in Ogden; his mother, Helen Hayden, of Akron, Ohio, and two sisters, Mary V. Keyser, of Wadsworth, Ohio, and Sandy Clark, of Dayton, Ohio.

The family suggests that expressions of sympathy may be in the form of contributions to The Nature Conservancy in Arlington.