Throughout the post-war period communist planners in Bulgaria pursued a strategy of rapid industrialization of an agrarian economy based on state ownership of industrial enterprises and a system of central planning. With the exception of some private activities in agriculture and widespread private-owner occupation in the housing sector, there was virtually no private economic activity in the economy. Industrialization was assisted by substantial financial and technological transfers from the former USSR, high domestic investment rates, and growing specialization in machinery exports into the protected market of the CMEA countries. However, the negative side of this progress became more and more evident in the 1980s: environmental degradation, difficulties in maintaining high economic growth, a rapid increase of the external debt in convertible currencies. According to one estimate (Minassian, 1992), industrial output based on a pattern of extensive development eventually stopped growing and began to fall from 1983 onwards.