ABSTRACTEstimations were made of the numbers of germ cells in the guinea-pig ovary from embryonic stages to maturity. A volumetric method was used to estimate the numbers of oögonia and Oöcytes in animals aged 26 to 55 days p.c., while counts were made of the numbers of germ cells present between 60 days p.c. and 12 months p.p. Neither the mitotic activity of oögonia nor the successive stages of meiotic prophase are closely synchronized. The process of oögenesis ceases at about 50 days p.c. Large numbers of Oöcytes are found in the leptotene stage at 35 days, in zygotene at about 40 days, and in pachytene at 50 days p.c. Thereafter the proportion of Oöcytes in the diplotene phase increases, so that by a few days after birth all the Oöcytes are in this stage. By 1 month p.p., however, two types of Oöcyte in diplotene are found in large numbers. The first is comparable to the corresponding stage in rat, man and the chick, while the second displays a very contracted nucleus. The latter type rapidly gains predominance, and by 12 months p.p. 94 per cent, of the germ cells belong to it. The number of germ cells per ovary increases from about 27,000 at 30 days p.c. to reach a peak of almost 105,000 by 41 days p.c.; subsequently it declines to some 13,000 by 12 months p.p. This decline is caused by a steady process of atresia which affects germ cells from the earliest stages of development.