IN A RECENT BOOK AND articles, one of the present authors and Miguel Sidrauski [6, 7] have studied a macroeconomic growth model which explicitly includes monetary and fiscal policy tools as distinct parameters. One of the basic ideas of this work was to follow out rigorously an account of the volume of investment originated by Keynes [14], Witte [18] and others. In this paper we present an attempt to measure econometrically some of the key parameters of that model for the American economy in the period 1953-1970.2 This paper consists of a review and extension of the theory of the determination of the volume of investment mentioned above, an account of our use of a spectral smoothing technique to handle an errors-in-variables problem inherent in our basic approach, and a report of our estimated equations for producer structures and durables over the period. We believe that these equations perform as well or better than those based on other theoretical constructs in terms of coefficient size and significance, goodness of fit, and reasonableness of lag structure. To facilitate comparison we have estimated our equations for the same period and data that Bischoff [1] used in his study of investment functions.