Foetal, neonatal and post-natal human ovaries (2 months p.c. to 7 years p.p.) were studied with the object of correlating cytological changes in the germ cells with fluctuations in their numbers. The chromosomal configurations of oogonia and oocytes were examined in both histological and squash preparations. The numbers of normal and degenerating germ cells at different developmental stages up to the time of birth were estimated volumetrically, while those in post-natal ovaries were assessed by means of cell counts. Mitotic divisions in oogonia were observed from the end of the 2nd up to the 7th month p.c., a period during which some oocytes had already entered the leptotene and zygotene stages of meiotic prophase. Oocytes at the pachytene stage were recorded from 4 months p.c. until the time of birth, and those at diplotene from the 4th month throughout the remainder of the period studied. The so-called dictyate stage was not observed in human oocytes. Three 'waves' of degeneration were observed, affecting (i) oogonia undergoing mitosis ('atretic divisions'), (ii) oocytes principally at the pachytene stage ('Z' cells) and (iii) oocytes at the diplotene stage. The total population of germ cells appears to rise steadily from some 600 000 at 2 months p.c., reaching a peak of 6 800 000 at 5 months. By the time of birth, the number has declined to 2 000 000 of which some 50% are atretic. Of the 1 000 000 normal oocytes in the new-born infant, only some 300 000 survive to the age of 7 years.