Fully grown Xenopus oocytes contain a stockpile of inactive Maturation Promoting Factor, commonly termed pre-MPF, and are arrested in interphase. The hormone progesterone releases oocytes from this arrest and induces 2 subsequent bursts of MPF activity. The first wave causes entry into meiosis I, the second burst of MPF activity induces meiosis II. Both of these events depend on protein synthesis, but the nature of the proteins required to accumulate is poorly understood. This thesis describes the role of B-type cyclin synthesis during oocyte maturation. Firstly, I have characterised expression patterns of the four different B-type cyclins that are present during meiosis. Two of these B-type cyclins have not been previously described. Secondly I have designed potent and specific antisense oligonucleotides to inhibit cyclin synthesis. Thirdly, I have studied the effects of inhibiting B-type cyclin synthesis on Xenopus oocyte maturation. I find that the first activation of MPF and entry into meiosis is not dependent on B-type cyclin synthesis. The stockpile of pre-MPF (consisting of cyclins B2 and B5) appears to be sufficient to allow completion of meiosis I. The second activation of MPF at the onset of meiosis II, on the other hand, is strictly dependent on new B-type cyclin synthesis, predominantly that of cyclins B1 and B4. This study clarifies the role of B-type cyclin synthesis during Xenopus oocyte maturation.