Thyroid hormones have diverse effects on ovarian function. We examined the expression of thyroid hormone receptor (TR) mRNAs (including TRalpha-1, TRbeta-1, TRbeta-2, and c-erbAalpha-2 isoforms) in three types of cells from human follicles, and determined the concentration of free tri-iodothyronine (T3) present in human follicular fluid. Human failed-fertilized oocytes, granulosa (GC) and cumulus (CC) cells from patients of the in-vitro fertilization (IVF) programme at Alliant Hospital Fertility Center were used to detect TR mRNA expression using reverse transcription-polymerase chain reaction (RT-PCR) followed by Southern blot analysis. Human spermatozoa were also analysed to determine whether results obtained with CC would be affected by the presence of spermatozoa. beta-Actin mRNA was amplified in each cell type as a positive control for the RT-PCR. Our results show that human oocytes express TRalpha-1, TRbeta-1, TRbeta-2, and c-erbAalpha-2 mRNAs and that these same isoforms are expressed in both human granulosa cells and cumulus cells. No differences were detected in the apparent amounts of RT-PCR products when comparing GC with CC, suggesting a similar pattern of expression of these RNAs. beta-actin mRNA was detected in spermatozoa, but TRalpha-1 expression was not detectable. The concentrations of free T3 measured in follicular fluid were similar to, or slightly below, those in serum of euthyroid patients. These data demonstrated that several isoforms of TR mRNA are expressed in the human oocyte, and hence thyroid hormone may have direct affects on the oocyte, as well as on GC and CC. In addition thyroid hormone may have indirect effects on the oocytes via the CC.