The qualitative profiles of the proteins synthesized by sheep oocytes at various stages of maturation were determined by electrophoretic separation in one dimension on polyacrylamide SDS gels. No change in protein synthetic pattern was observed in ooce changes had taken place in at least 12 separate protein bands. Marked alterations in the synthesis of some proteins were apparent 15 h after LH; formation of proteins in 5 of the original bands was either reduced or not detectable, while new synthesis was evident from the appearance of 7 additional bands. The pattern of proteins produced by oocytes cultured within the follicle corresponded closely with that observed in vivo: changes in synthesis were initiated about 9 h after addition of gonadotrophin and were completed by 15 h. Oocytes cultured outside the follicle in a gonadotrophin-containing medium did not exhibit a change in protein synthesis and at 15 h only those proteins produced during the early stages of maturation were being synthesized.