The effect of immunoneutralization of of gonadotropin-releasing hormone (GnRH) on LH secretion and concentrations of GnRH receptor, GnRH receptor mRNA, and gonadotropin subunit mRNA in pituitary tissue of orchidectomized sheep (wethers) was assessed. Thirty-six wethers were assigned at random to one of six treatment groups (six wethers per group). Thirty wethers (groups 2 to 6) received 200 mL ( i. v. ) of anti-GnRH antisera at passive immunization (PI). Anterior pituitary tissue was collected 0.5, 1, 2, 4, or 8 d after PI from wethers in groups 2 to 6, respectively. Pituitary tissue was also collected from unimmunized wethers (Group 1). Intravenous administration of anti-GnRH sera increased anti-GnRH activity to 69.1 +/-0.7% (percentage of total 125I-labeled GnRH bound by a 1:1,000 serum: GEL-PBS dilution) within 1 h of PI. Anti-GnRH activity declined gradually during the period after PI, and 8 d after PI anti-GnRH activity was 57.2 +/- 1.7%. Serum concentration of LH was significantly reduced, relative to the pretreatment (16.1 +/- 1.8 ng/mL) level, within 4 h (7.6 + 1.5 ng/mL) of PI, and the LH level was 10% of the pretreatment concentration 8 d after PI (1.6 + 0.2 ng/mL). Steady-state concentration of GnRH receptor mRNA decreased progressively during the period after PI and was significantly reduced, relative to the level in unimmunized control wethers (.44 +/- 0.03 pg/micrograms total RNA), 4 (.33 +/- 0.03 pg/micrograms total RNA), and 8 (.31 +/- 0.02 pg/micrograms total RNA) d after PI. Tissue concentrations of GnRH receptor and mRNA for the alpha LH beta, and FSH beta, subunits were also reduced ( P 0.05) by PI. These data indicate that maintenance of steady-state concentrations of GnRH receptor and GnRH receptor mRNA requires continued GnRH stimulation