THE first procedure described for the quantitative determination of urinary pregnanediol, the principal excretory product of progesterone, was that of Venning (1) which depends upon the extraction, purification and gravimetric measurement of pregnanediol glucuronide as the sodium salt (NaPG). Although this method was carefully worked out by its originator from a quantitative aspect and has given curves of excretion that appear to be of physiologic significance in late pregnancy, it is open to a number of objections, even when applied to pregnancy urine of high pregnanediol titer, and has been found to be of limited value in early pregnancy and in the nonpregnant woman in whom the amount of pregnanediol excreted is below the level of quantitative accuracy or specificity of the method. Other methods commonly used since 1941 are based on the procedure developed by Astwood and Jones (2). This utilizes the fact that pregnanediol, freed from the glucuronide conjugate by hydrolysis, may be precipitated out of 95 per cent ethyl alcohol by the addition of 4 volumes of water. As originally described, the purified precipitate was weighed. The method was rendered more sensitive by Talbot et al. (3) who, in the final measurement of the purified pregnanediol, took advantage of the strong yellow color which this compound yields with concentrated sulfuric acid.