Recent reports suggest that androgens increase FSH transcription directly via the androgen receptor and by modulating activin-signaling. Estrogens may also regulate FSH transcription in part through the activin system. Activin signaling can be regulated extracellularly via activin, inhibin, or follistatin (FS) or intracellularly via the Smad proteins. We determined the effects of androgen and estrogen on FSH primary transcript (PT) concentrations in male and female rats and we correlated those changes with pituitary: activin B mRNA, FS mRNA, the mRNAs for Smads 2,3,4, and 7, and the phosphorylation (p) status of Smads 2 and 3 proteins. In males, testosterone (T) increased FSH PT 2-3 fold between 3-24h and was correlated with reduced FS mRNA, transient increases in Smads 2, 4, and 7 mRNAs, and a 6-10 fold increase in pSmad2, activin B mRNA was unchanged. In females, T also increased FSH PT 2-fold, pSmad2 3-fold, but had no affect on activin B, FS, or the Smad mRNAs. Androgen also increased Smad 2 phosphorylation in gonadotrope derived T3 cells. In contrast, estradiol had no effect on FSH PT, but transiently increased activin B mRNA and suppressed FS mRNA before increasing FS mRNA at 24h, increased Smads 2,3, and 7 mRNAs, and pSmad2 3-fold. In conclusion, T acts on the pituitary to increase FSH PT in both sexes and modulated FS mRNA, Smad mRNAs, and/or Smad2 phosphorylation. These findings suggest that T regulates FSH transcription, in part, through modulation of various components of the activin signaling system.