Marshall & Jolly (1905 a, b) were the first to report that removal of the ovaries from the rat early in pregnancy was followed by foetal loss, although the duration of pregnancy when the rats were spayed is unknown. Subsequently, a number of workers have carried out similar experiments at different stages of pregnancy with or without removal of some of the foetuses (Selye, Collip & Thomson, 1935;Haterius, 1936;McKeown & Zuckerman, 1938;Zeiner, 1943).In all this work, the number offoetuses present in the rats were veryvariable, and it was thought desirable that a standard number of young should be left in the mother at the time of spaying. The present work was simplified by removing both ovaries simultaneously. In order to clarify the effects of the operation, it appeared necessary to spay one group of mother rats early in pregnancy and compare the subsequent findings with those after spaying at a relatively late stage. The number of foetuses in each pregnant rat was accordingly reduced to six on the 9th day as a standard procedure to obtain what is hereafter called the 'standardized' pregnant rat (Alexander & Frazer, 1954). Some rats were also spayed at this time, i.e. before the formation of the chorio-allantoic placenta, and these were compared with a similar series in which the operation of spaying was performed only 4 days before term.In rats spayed on the 9th day, it was expected that resorption of the foetuses might follow. If this should be the case, such a spayed rat would be a useful test animal to ascertain whether progesterone alone was sufficient to maintain pregnancy; and, if successful, whether other hormones and substances would also keep the foetuses alive. The ability to maintain pregnancy in an animal which would lose its foetuses in the absence of progesterone would certainly reveal any agent which had the essential properties of the latter.