The pharmacokinetics of inert gases have been of 1962) and during uptake (Salanitre et al., 1962). great interest to respiratory physiologists at least since the work of Bert (1878), but the study of gas exchange has particularly accelerated since the stimulating review and analysis by Kety (1951). Nitrous oxide has been extensively studied, both because of its importance as an anaesthetic and because its solubility characteristics and relative physiological inactivity have made it so useful in the study of the circulation. Each of these applications of the kinetics of nitrous oxide have been reviewed recently (Epstein, 1963; Eger, 1964) as has the general pharmacology of nitrous oxide (Hamilton, 1963). The paper by Eger, to which the reader is referred, is particularly complete in analyzing the volumes of gas involved in production of nitrous oxide anaesthesia and the role of anaesthetic circuits and techniques of administration in determining the rate of nitrous oxide exchange. The data which have accumulated on the overall rates of nitrous oxide exchange are of particular interest because they permit a direct test of the success of theories of inert gas exchange in accounting for experimental observation. In this sense, nitrous oxide is of interest not only for its own importance as an anaesthetic but as a prototype of inert gases generally. This paper will review studies of nitrous oxide uptake and excretion which have supported basic theories of inert gas kinetics and others which have required specific modification of these theories. The total body uptake of nitrous oxide has been studied in several ways. The absolute volumes of gas absorbed during closed system nitrous oxide anaesthesia were determined by Severinghaus (1954). The rate of approach to equilibrium following an abrupt alteration of the inspired nitrous oxide concentration was measured both during excretion (Frumin et al., 1961; Salanitre et al., 172