In vivo tracer experiments involving intravenous infusion of 1, 2 or 3 labelled hormones are described in schematic form. Formulae are provided to calculate from steady state isotopic data, rates of production and interconversion of circulating compounds, foeto-maternal production and transfer of hormones and rates of metabolic reactions occurring in an organ. Formal procedures to convert these formulae to expressions applicable to single injection experiments are outlined. The simplicity of the mathematical basis for the interpretation of most current experiments in hormone research is emphasized.