1. A review is given of some phenomena concerning pregnancy and parturition of the Cetacea, depending on data in literature and on observations made in Antarctic Blue and Fin Whales on board the f.f. “Willem Barendsz” (1946—1947). 2. In Mystacoceti the frequency of twins appears to be less than in man and the big domestic animals. The majority of twins is born by females that are longer than the average female in the period of greatest sexual activity. This does not mean, however, that just as in man most twins are born at an elder age than that corresponding with the maximum of sexual activity. It may also be possible that, just as in the big domestic animals, the ages correspond, but that twins are mostly produced by the physically stronger developed females. 3. In Odontoceti the left ovary shows a very distinct morphological and functional prevalence. The foetus is exclusively found in the left uterine cornu. In Mystacoceti there is a prevalence of about 60 % of the right ovary and the right uterine cornu with regard to ovulation and pregnancy. The above described phenomena have also been observed in other uniparous Mammals, whereas in multiparous Mammals no distinct prevalence of a special side has been found. Transference of an ovum from the ovary of one side to the cornu of the other has been observed twice in Cetacea. 4. In Mystacoceti the number of cephalic presentations of the foetus very distinctly increases during the last months of pregnancy, just as in man and the big domestic animals. There is a great possibility that, just as the other uniparous Mammals, which give birth to comparatively large infants, in Mystacoceti a very high percentage of the young is born in cephalic presentation. In Odontoceti, on the contrary, a great number of foetuses is apparently delivered in tail-presentation. With regard to their shape and dimensions, such a birth in tail-presentation must be considered as an unfavourable event. An attempt has been made to explain these facts with the aid of the peristaltic uterine contractions. If these contractions act in the same way as in other Mammals, it might be expected that most of the Cetacea should be born in tailpresentation. So it is highly possible that in Mystacoceti some other factors are responsible for the high percentage of cephalic presentations. 5. In Cetacea the relative length of the umbilical cord (in % of the length of the foetus) decreases markedly during the second part of pregnancy. At birth its length is about 40 % of the total length of the calf and 57 % of its snout-anus length. As compared with other Mammals the cord of the Cetacea is rather short and this fact may have some influence on the way in which the connection between mother and calf is broken. This may occur by rupture of the cord immediately after birth as in Ungulates. On the other hand it appears to be also possible that, just as in Primates, Carnivores and Chiropteres, the placenta and the cord stick to the baby for some time after birth. 6. Some congenital anomalies of foetal Cetacea are described. Abortus probably may occur during chasing of the big whales or when they are struck by the harpoon. A case of fibromyoma uteri is described in an old female Blue Whale that showed an abnormal lactation. Probably the fibromyoma had caused an abortus some months ago, this abortus causing the lactation.