Introduction Anyone who studies human ketosis with some intensity becomes impressed, and sometimes exasperated, with the unsatisfactory chemical methods he has to use; with the quantitative variability of the effects of any given ketogenic regimen among different subjects and within the same subject from time to time, and with the difficulty of fitting his results into a unified set of postulates. This paper is based mainly on work at Illinois on nutritional and environmental factors in human ketosis, and on a series of studies on postexercise ketosis at Edinburgh, Scotland. We shall touch briefly on nomenclature and methodology, spend most of the time on multiple factors which can affect the severity of experimental ketosis in humans, and finally speculate on how these multiple factors may be fitted into some current concepts on endocrinology and intermediary metabolism. Nomenclature Although the term ketosis may still be useful clinically, by any reasonable philological