Back in The Netherlands he worked with the architects Aldo van Eyck and Gerrit Rietveld. He likewise fostered and maintained his many international contacts, which also included those with the Danish artist Asger Jorn. Constant became ever more convinced that a new social and cultural order will advance the cause of humanity. His visionary spirit anticipated that the automation of production will provide mankind with increasing amounts of free time (thereby creating scope for the “Homo Ludens”, the man who is playful and creative) and that this futuristic view would become feasible in what, from 1960, was to be known as New Babylon. This vision would dominate his oeuvre from that moment on making drawings, models, photo collages, plans, constructions and paintings. In addition, he substantiated New Babylon with numerous written works, talks and lectures in various countries.The social and political turbulence of the sixties threatened to turn New Babylon into a utopia: the New Babylon project stopped in 1974.

Like so many artists worldwide, Constant was deeply concerned by the Vietnam war, changing his artistic direction into a more revolutionary and political way. He created collages and works on paper, as well as impressive paintings which he will continue to do during the Balkan war of the early nineties.

Happening was exhibited at:

Paris, Galerie Daniel Gervis, Constant, peintures recentes, 30 October -30 November 1974;

Amsterdam, Galerie Collection d' Art, Schilderijen en aquarellen , Constant, 20 March- 29 April 1976;

Amsterdam, Stedelijk Museum, Constant, Schilderijen 1969-77, 17 March – 7 May 1978, no. 9 (ill.cat.);

The Hague, Gemeentemuseum, Constant Schilderijen 1940 – 1980, 27 September – 24 November 1980 (sticker on the stretcher, not in catalogue);

Antibes, Musée Picasso, Constant, une rétrospective , 30 June- 15 October 2001, no. 56 (colour ill.cat. on p. 108).