For 15 years, he worked as an accompanist at a music school, and composed pieces for foreign commissions that he could not hear being performed.

He read books such as Ctirad Kohoutek's New Compositional Theories of Western European Music (Prague 1962), listened at the Warsaw Autumn to music and met Czech composers, Witold Lutosławski, Krzysztof Penderećki and others, and had personal contacts with Western European composers including Luigi Nono and Karlheinz Stockhausen.

[3] In the 1960s, Kopelent became well known in contemporary European music circles, with his compositions being performed at such festivals as Warsaw Autumn, Donaueschinger Musiktage, Wittener Tage für neue Kammermusik and the annual Darmstädter Ferienkurse.

[5] From 1965 to 1973, Kopelent served as an artistic director of the contemporary music ensemble Musica Viva Pragensis,[3] which had been founded by Petr Kotík in 1961.

[7] He was ostracized by the new Union of Composers, and his ensemble Musica viva Pragensis was not permitted by the authorities to pursue its concert activity.

[1][5] In 1976, Kopelent accepted a job as a piano accompanist for a children's dance schools in Radotin, where he remained for 15 years.

[1][2] During the 1970s he composed many pieces, a number of them for foreign commissions, but, as he could not leave Czechoslovakia, he was unable to hear their performances.

[8] In 1991 he was appointed professor of composition at the musical faculty of Academy of Fine Arts in Prague, a position he retained.

[8][10] Kopelent was the organiser and a regular lecturer to International Composers' Summer Courses, held in Český Krumlov.