THE use of the spayed rat’s uterus as an end-point for estrogen assay was suggested by Bülbring and Burn (1935). Dorfman et al. (1936) and Lauson et al. (1939) used the uterine response of the intact immature rat for estrogen assay. Neither of these studies nor the studies of Astwood (1938) who described a 6-hour test and also used the intact immature rat uterus contained statistical analysis of the data. Emmens (1950) calculated the precision of the data of Lauson et al. (1939) and found that if a total of 40 rats are used in a 4-point assay, the expected minimal limits of error are approximately (P = 0.95) 81 to 124%. This report was designed to study the utility of immature rat uteri as an end-point for the quantitative bioassay of estrogens using both the subcutaneous and oral routes of administration and to study the relative oral activity of various estrogen sulfates.