In New York City, Morris began to explore the work of Marcel Duchamp, making conceptual pieces such as "Box with the Sound of its Own Making" (1961) and "Fountain" (1963). In 1963 he had an exhibition of Minimal sculptures at the Green Gallery in New York that was written about by Donald Judd. The following year, also at Green Gallery, Morris exhibited a suite of large-scale polyhedron forms constructed from 2 x 4s and gray-painted plywood. In 1964 Morris devised and performed two celebrated performance artworks "21.3" in which he lip syncs to a reading of an essay by Erwin Panofsky and "Site" with Carolee Schneemann. Morris enrolled at Hunter College in New York (his masters thesis was on the work of Brâncuși) and in 1966 published a series of influential essays "Notes on Sculpture" in Artforum. He exhibited two "L Beams" in the seminal 1966 exhibit, "Primary Structures" at the Jewish Museum in New York.