By the 1960s Yamaguchi was experimenting with more sculptural works and experimenting with the contingent relationship between object and space. On a 1961 visit to New York, he met Frederick Kiesler at his studio, which led him to produce a fifteen-part series of articles on Kiesler's life and work for the major art magazine "Bijutsu Techō" between January 1976 and March 1977, eventually published in book form in 1978. He began producing wire sculptures wrapped in sack cloth from commercial product packaging, magnetic sculptures with re-arrangeable parts, and . At this time he was active in several groups including the Environment Society (Enbairamento no Kai), which put together the 1966 exhibition "From Space to Environment", and the Japan Electric Arts Association (Nihon Denki Geijutsu Kyōkai), through which he organized the 1969 exhibition "Electromagica ’69: International Psytech Art". He also occasionally participated in Fluxus-related events, such as the 1966 "Happening for Sightseeing Bus Trip in Tokyo" and "Expose ’68: Say Something Now, I’m Looking for Something to Say". During the latter half of the decade, Yamaguchi began to articulate an idea of expanded sculpture in his 1967 book "Futeikei Bijutsu-ron" (Theory of Indeterminate Art), exhibited in the Venice Biennale (1968), and became commissioner for the Mitsui Pavilion at Expo ’70.