Follicular development and function were compared in 12-16-week-old Romanov and non-prolific (Ile-de-France or Prealpes) lambs. At 12 weeks of age, there was no difference between breeds in the population of antral follicles, their size distribution, their atresia, or the size of the largest follicles. Whatever the breed, intact follicles explanted from 12-week-old lambs actively produced testosterone in an unsupplemented medium. This production increased in a gonadotrophin-supplemented medium. In contrast, basal oestradiol production by Romanov follicles was reduced compared with Ile-de-France and Prealpes follicles. However, in both breeds this production increased about two-fold in an FSH-supplemented medium. At 14-16 weeks of age, lambs of both breeds had LH-sensitive follicles, as evidenced by their ability to ovulate following the injection of hCG. The number of such follicles was related to the prolificacy of the lambs (Romanov 2.8, Ile-de-France 1.6). Hence, at 12-16 weeks of age, the maturation of the ovary is not fully completed in Romanov lambs. Acquisition of a fully functional aromatase system and maturation of the central nervous system are probably required for puberty to proceed. Their relative timings may differ between breeds.