Despite no evidence for desensitization of phospholipase C-coupled gonadotropin-releasing hormone (GnRH) receptors, we previously reported marked suppression of GnRH-mediated Ca 2؉ responses in ␣T3-1 cells by pre-exposure to GnRH. This suppression could not be accounted for solely by reduced inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) responses, thereby implicating uncoupling of Ins(1,4,5)P 3 production and Ca 2؉ mobilization (McArdle, C. A., Willars, G. B., Fowkes, R. C., Nahorski, S. R., Davidson, J. S., and Forrest-Owen, W. (1996) J. Biol. Chem. 271, 23711-23717). In the current study we demonstrate that GnRH causes a homologous and heterologous desensitization of Ca 2؉ signaling in ␣T3-1 cells that is coincident with a rapid (t1 ⁄2 < 20 min), marked, and functionally relevant loss of type I Ins(1,4,5)P 3 receptor immunoreactivity and binding. Furthermore, using an ␣T3-1 cell line expressing recombinant muscarinic M 3 receptors we show that the unique resistance of the GnRH receptor to rapid desensitization contributes to a fast, profound, and sustained loss of Ins(1,4,5)P 3 receptor immunoreactivity. These data highlight a potential role for rapid Ins(1,4,5)P 3 receptor down-regulation in homologous and heterologous desensitization and in particular suggest that this mechanism may contribute to the suppression of the reproductive system that is exploited in the major clinical applications of GnRH analogues.