When environmentalists from the industrialized countries first urged third world leaders to protect their natural resources, the advice was viewed with considerable skeptism. Some in the lesser developed regions insisted that ''poverty is the worst pollution'' and suspected a plot to keep them in an economically dependent role.
In recent years, however, that attitude has changed significantly as one country after another has taken official steps to ensure that resources are adequately protected. Nine years ago, only 11 developing countries had environmental ministries. The figure now has risen to 102.
To underscore this concern, the World Bank has begun insisting that environmental protection be built into new third world projects before it will lend out money.
In his first major speech on the subject, World Bank president A. W. Clausen Nov. 12 emphasized the ''global imperative'' of development that includes ''vigorous attention to resource management and the environment.''
Mr. Clausen is thus following the lead of his predecessor, Robert McNamara, who retired from the World Bank presidency in July. Clausen's address to the Conservation Foundation in Washington, D.C., was welcomed by environmentalists, many of whom have found unsettling earlier pronouncements from the Reagan administration about economic development and environmental issues.
''Conservation, in its broadest sense, is not a luxury for people rich enough to vacation in scenic parks,'' Clausen said. ''Rather, the goal of economic growth itself dictates a serious and abiding concern for resource management.''
To ensure that this concern is addressed, the World Bank's environmental staff now reviews more than 200 projects a year. The agency's loan program for water supply and waste disposal has more than doubled since the mid-1970s, and loans for forestry projects have increased sixfold in recent years. Such projects range from the relatively simple to those that are massive and complex.
In Burundi, where many people spend 40 percent of their income on firewood and fuel, the bank has financed a program to make more efficient cooking stoves available. Loans for a large iron ore project in Brazil that included a railroad and new port facility were granted only after protections for forests, water supplies, air quality, and coastal areas were guaranteed.
As a penny-wise banker, Clausen also stresses the cost-effectiveness of addressing environmental problems before industrialization takes place. World Bank officials have found environmental costs make up only 3 to 5 percent of a new project's overall cost.
''We're convinced that it's almost always less expensive to incorporate the environmental dimensions into project planning than to ignore them and pay the penalties at some future time,'' Clausen said.
Protecting the environment in developing countries remains an enormous problem. Third world forests are being cut down 10 times as fast as they are being replanted, and the Earth has lost one-fourth of its rain forests as a result. Intensive cultivation and overgrazing have led to ''desertification'' in parts of the third world.
''But,'' says Clausen, ''awareness is spreading that environmental precautions are essential for continued economic development over the long run.''