1. In the pregnant guinea pig proliferation of the mammary gland becomes regular only at a later stage of pregnancy; namely, during the period following the 24th day of pregnancy. Previous to this period proliferation was absent in the majority of cases. Proliferation of the mammary gland during pregnancy becomes regular only at a period of time which exceeds the duration of the normal sexual cycle unaccompanied by pregnancy. It is probable that pregnancy as well as the presence of living deciduornata and corpora lutea increases the proliferative activity of the mammary gland as compared with the ordinary cycle in non-pregnant animals or in animals lacking corpora lutea and deciduornata.

2. After the completion of pregnancy and in the beginning of secretion some mitotic proliferation may still be present, but it soon ceases, probably as the result of those processes that lead to secretion. While during the period of secretion, notwithstanding the presence of a new pregnancy, mitotic proliferation soon ceases, some proliferative stimulus seems still to be active, which, however, under existing conditions apparently leads only to a mitotic multiplication of nuclei. The latter conclusion is only suggested at the present time and needs confirmation through further studies.

3. In cases in which abortion took place in the first half of pregnancy secretion in the gland was not established; secretion occurred in two animals aborting toward the latter part of pregnancy. In one of these cases, mitotic proliferation of some gland cells was associated with the microscopic appearances of secretion.

4. In guinea pigs castrated during an early period of pregnancy in which pregnancy continued for some time, proliferative changes were absent in the mammary gland. In conjunction with a partial similar effect observed after extirpation of the corpora lutea during pregnancy, we may perhaps attribute the lack of proliferation in some of these cases to the absence of the ovaries.

5. Extirpation of the corpora lutea during pregnancy induces a new ovulation and with it the primary proliferation in the mammary gland; abortion does not necessarily prevent these proliferative changes. Extirpation of the corpora lutea during pregnancy perhaps prevents the secondary proliferative changes in the mammary gland.

6. Five injections of cow's lutein given in relatively large quantities intraperitoneally do not produce proliferation of the mammary gland in the guinea pig.