The experiment was performed to confirm selective stimulation of FSH release by GnRH analog 4-MeSer-GnRH and to determine if analog can act on α, LHβ and prolactin gene expression. Our studies in vivo have shown that 4-MeSer-GnRH FSH releasing potency was less expressed than it was shown in our previous experiments and that this peptide did not affect LH release. GnRH analog inhibited both pituitary prolactin and LHβ mRNAs level but this effect was much more strongly expressed in case of prolactin mRNA. Such data point out a necessity of more detailed studies on the biological properties of 4-MeSer-GnRH.