In order to examine the effects of thioglycolic acid (TGA) on reproduction, Xenopus oocytes were treated with different concentrations of TGA. During culture, frequencies of germinal vesicle breakdown (GVBD) and MI-MII transition were determined. Samples collected at indicated times were subjected to immunoblotting. Data indicated that TGA accelerated the frequency of GVBD, but inhibited polar body extrusion and formation of MII-arrested eggs in a concentration-dependent manner. At 4 h after progesterone addition, phosphorylation of extracellular signal-regulated kinase (ERK) and p90 ribosomal S6 kinase, two members of the mitogen-activated protein kinase (MAPK) pathway, was upregulated in TGA-treated oocytes. The regulatory subunit of M-phase promoting factor (MPF)-cyclin B was also upregulated by TGA, while phospho-Cdc2 was downregulated. At 8 h, Cdc2 dephosphorylation and cyclin B1 were downregulated by TGA treatment. However, TGA exerted no effect on Mos, an MAPKKK (MAPK kinase kinase). In conclusion, TGA has the potential to inhibit in vitro maturation of Xenopus oocyte with increased GVBD frequency accompanied by alterations in protein expression and phosphorylation involved in MPF and MAPK pathways. Since egg formation is essential to maintain appropriate reproductive capacity, our findings may have certain toxicological implications.