The nominal efficiency of a regulatory regime is determined by comparing its social costs and benefits; the regime is nominally efficient if it produces benefits in excess of its costs. Thus, a regulatory regime can be at once nominally efficient and relatively inefficient. A regulatory regime that is nominally efficient in the early days of pollution-control efforts, when increments of environmental quality are relatively cheap, may (but will not necessarily) grow less efficient over time - producing less return on each dollar invested - as increments of environmental quality grow increasingly expensive. A regulatory regime that is more efficient in one institutional and technological setting may be less efficient (or inefficient) in another. In reality, however, this outcome will occur only under certain conditions; specifically, when the regulatory regime as a whole is more efficient. The discussion begins, for the sake of comparison, with a brief review of the "conventional" story of the Clean Air Act's regulatory regime. In addition, in 1987 the EPA wrapped up a small-scale and temporary but highly successful experiment in tradable rights to lead-content in gasoline. Like other institutions in society, those of environmental protection (including the regulatory regime itself) tend to evolve slowly, incrementally, and inconsistently. In large measure, the choice of regulatory regime depends on the goals and concerns of policy-makers.