A novel series of derivatives of mono- and diaminopyrimidines 1 potently displaced binding of a radiolabeled GnRH analogue to human and rat GnRH receptors. Analogues from these series competitively antagonized GnRH-stimulated increases in extracellular acidification in vitro and suppressed GnRH-mediated increases in circulating luteinizing hormone (LH) in castrated rats and testosterone in intact rats. These compounds or their analogues may be useful in treating sex hormone-dependent disease.