During the early preimplantation stages of human embryos, pyruvate and lactate, but not glucose, are the preferred energy substrates. Transport of these monocarboxylates is mediated, in mammalian cells, by a family of transporters, designated as monocarboxylate transporters (MCTs). Human and mouse genetic expression of MCT members 1, 2, 3, 4 and basigin, a chaperone protein of MCT1 and MCT4, was qualitatively analysed using the reverse transcription nested polymerase chain reaction (RT-nested PCR) in immature oocytes (germinal vesicle stage; GV), in non-fertilised metaphase II (MII) oocytes and in embryos from 2-cell stage to blastocysts. Transcripts encoding for MCT1 and MCT2 were present, under a polyadenylated form, in the majority of the human and mouse oocytes and early embryos. MCT3 transcripts were not detected in either human or mouse. MCT4 mRNA was not detected in human oocytes and embryos, but was present in mouse oocytes and embryos. This fact could imply differences in lactate transport and regulation of intracellular pH between human and murine early embryos. Basigin transcripts were present in mouse and human MII oocytes and preimplantation embryos, but were not detected at GV stage. However, using 3' end-specific primers in the RT reaction instead of Oligo(dT)12-18 primers, transcripts encoding for this protein were then detected at GV stage in both species. This result suggests that a regulated polyadenylation process occurs during oocyte maturation for these transcripts. Thus, basigin mRNA can be considered as a marker of oocyte cytoplasmic maturation in human and mouse species.