The present study was undertaken to identify the mechanisms underlying the effect of transforming growth factor (TGF) β on FSH receptor (FSH-R) in rat granulosa cells. Compared to the control, the treatment of granulosa cells with TGFβ (10 ng/ml) increased FSH-R mRNA transcripts (5.5 and 2.4 kilobases) in a time-dependent manner, with a maximum increase of approximately 2-fold at 48 h. We then investigated whether the effect of TGFβ on FSH-R mRNA levels was the result of increased transcription and/or altered mRNA stability. To determine whether the FSH-R 5′-flanking region plays a role in directing FSH-R mRNA expression, the proximal area of the FSH-R 5′-flanking regions were inserted into an expression vector, pGL-Basic, which contains luciferase as the receptor gene, and the resulting plasmids were transiently transfected into rat granulosa cells. The FSH (30 ng/ml) significantly enhanced the activity of 1862 base pairs of the FSH-R 5′-flanking region, but treatment with TGFβ did not significantly influence the activity induced by FSH. On the other hand, the decay curves for FSH-R mRNA transcript in primary granulosa cells showed a significant increase in half-life after the addition of TGFβ. Transforming growth factor β stimulates the expression of follistatin mRNA accumulation in a dose- and time-dependent manner. Treatment with activin produced a substantial increase in FSH-R mRNA level. Concurrent treatment with follistatin neutralized this activin effect on FSH-R mRNA, as reported, although concurrent treatment with follistatin did not affect TGFβ-induced FSH-R mRNA. Therefore, the profile of the TGFβ effect on FSH-R mRNA granulosa cells may be caused by the increased stability of FSH-R mRNA and insensitivity to the follistatin.