ABSTRACT Specific activity curves of circulating compounds, labeled by intravenous injection of tracers, are used to obtain quantitative information about the rates at which blood-borne compounds are produced and metabolized in vivo. Most of the useful parameters calculated from tracer experiments (production rates, metabolic clearance rates, rates of conversion between circulating compounds, feto-maternal transfers) are obtained by measuring the areas enclosed by the specific activity curves. Patterns of distribution and metabolism of the compounds need not be assumed. Additional information is provided by the “shape” of the specific activity curve of the circulating compound, e.g., the amount of the compound in the space of fast initial distribution of the tracer, total rate of exit from this space, fraction re-entering the circulation, mean transit time. Again, these parameters are calculated without postulating models to describe the distribution and metabolic fate of the compound. The usual approach ...