The maturation of the secondary follicle population from birth to 6 months is traced by comparing the ratio of mature fibre-producing secondary to primary follicles at successive ages, with the ratio at birth of all secondary follicles, mature and immature, to primary follicles. In lambs of various sheep types all secondary follicles were present at birth. The rate at which secondary follicles mature reaches a maximum at 7-21 days after birth, with some 65.0 per cent. of secondaries growing fibres by 28 days. The evidence indicates that the adult secondary follicle population may be permanently affected in two ways. Firstly, the number of secondary follicles initiated up to the time of birth may be restricted by adverse foetal environments. Secondly, the number of initiated secondary follicles which mature may be restricted by the early post-natal environment, particularly up to 21 days.