The development of preovulatory follicles involves an initial phase of somatic cell differentiation and a final phase, initiated by the LH surge, when both the somatic and germinal compartments alter. Abnormalities in this pattern of compartmental development after superovulation have been identified by examining follicles from control, PMSG- and FSH-treated sheep. The pattern of proteins synthesized by oocytes from untreated sheep did not differ after culture of follicles in hormone-free medium from that of germinal vesicle oocytes in vivo. Similarly, 93.5% of oocytes from sheep injected with a pituitary gonadotrophin (FSH-P) synthesized the unchanged germinal vesicle pattern of proteins during culture in an hormonally neutral culture environment. By contrast, the administration of the placental gonadotrophin, PMSG, induced in 28% of oocytes changes in the pattern of synthesis which are normally associated with maturation. An examination of follicular steroidogenesis showed that both total output and particularly oestrogen secretion was over twice as high in follicles from PMSG-treated as compared with FSH-treated animals (P less than 0.01). We conclude that the compartmental pattern of development and steroidogenesis is grossly perturbed in many follicles from PMSG-treated animals. Premature activation of the germinal compartment results in aged or abnormal oocytes and a hostile reproductive tract.