: OBJECTIVES: The purpose of this study was to examine whether in long-term ovariectomized rats direct pulsatile intracerebroventricular microinfusions of GnRH would result in frequency dependent biosynthesis of pituitary FSH beta-subunit mRNA. METHODS: Stainless steel cannula was stereotaxically implanted into the third ventricle of ovariectomized rats. 1 nM of GnRH microinjections were given at frequency 1, 2, 4 pulses/hour during 5 hours. Pituitary FSH beta-subunit mRNA level was determined by Northern-blot and serum FSH concentration was examined by RIA. RESULTS: Exogenous GnRH (1nM) induced a significant increase of pituitary content of FSH beta-subunit mRNA when administered 30 or 60 min intervals over 5 hours to ovariectomized rats. RESULTS: GnRH microinjections given at frequency of 1 pulse/hour were mostly effective resulting both in 85% increase of FSH beta-subunit mRNA level as compared to control as well as a significant (78%) stimulation of FSH release. CONCLUSIONS: our data show that in a long term ovariectomized rats (without steroid and gonadal peptides supplementation) a direct pulsatile intracerebroventricular microinjections of GnRH induce frequency-dependent pituitary FSH beta-subunit mRNA biosynthesis in the same mode as it was reported for gonadectomized and steroid replaced rats. It seems therefore that this method would represent an interesting alternative to the use of classical agents to disconnect, in vivo, the pituitary from hypothalamic GnRH influence. It may also provide a physiological data concerning regulatory aspects of gonadotropin subunits biosynthesis.