The present study was undertaken in order to identify the mechanism underlying the effect of transforming growth factor-β (TGFβ) on LH receptor (LH-R) expression in rat granulosa cells. Treatment with FSH produced a substantial increase in LH-R mRNA level, and concurrent treatment with increasing concentrations of TGFβ brought about dose-dependent increases in FSH-induced LH receptor mRNA. TGFβ, either alone or in combination with FSH, did not affect intracellular cAMP levels. We then investigated whether the effect of TGFβ and FSH on LH-R mRNA levels results in increased transcription and/or altered mRNA stability. To determine whether the LH receptor 5′-flanking region plays a role in directing LH receptor mRNA expression, the proximal area of the LH receptor 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. FSH (30 ng/ml) significantly enhanced the activity of 1389 base pairs of the LH receptor 5′-flanking region, but treatment with TGFβ did not significantly influence the activity induced by FSH. On the other hand, the decay curves for LH-R mRNA transcript in primary granulosa cells showed a significant increase in half-life after the addition of TGFβ.