We examined the qualitative patterns of protein synthesis in fully grown prophase-blocked oocytes of Xenopus laevis and after meiosis reinitiation accompanying maturation of the oocytes. Newly synthesized proteins labelled with [ 35S]methionine were run on isoelectric focusing gels and further separated in the second dimension on SDS-polyacrylamide slab gels. Three types of maturation inducer were compared: progesterone, considered as the natural inducer of Xenopus oocyte maturation, hCG (human chorionic gonadotropin) and insulin. Three polypeptides with apparent molecular masses of 37 kDa (p I 4.7–4.8), 78 kDa (p I 4.7) and 138 kDa (p I 4.6–4.7) were found to be always synthesized in all three types of stimulation, while the synthesis of a fourth one (molecular mass 116 kDa, p I 4.7) was arrested during oocyte maturation. Moreover, when the follicular cells surrounding the oocytes were part of the stimulating pathway, which is the case during hCG-induced maturation, an additional polypeptide was synthesized by the oocytes (molecular mass 106 kDa, p I 6.0–6.2). This polypeptide was not synthesized during progesterone- or insulin-induced oocyte maturation, two types of stimulation which do not require the presence of the follicular cells. The biological significance of the hCG-induced polypeptide, not necessary for oocyte maturation, is discussed. On the other hand, the four other modifications in protein synthesis taking place during all three types of maturation-inducing stimulation appear to be necessary for oocyte maturation, since oocytes which failed to mature in response to stimulation always missed one or several of these four polypeptides.