Pituitary tumorigenesis is now generally regarded as a multistep process of genomic damage leading to uncoupling of interdependent systems that control cell proliferation and differentiation. The alterations include mutations in genes encoding for proteins involved in signal transduction pathways, such as G-proteins and the p21 protein encoded for by the ras genes. Apart from their excessive secretion of ACTH, corticotropic adenomas are characterized by decreased sensitivity to inhibition by glucocorticoids. Therefore, mutations in the glucocorticoid receptor leading to decreased sensitivity to glucocorticoids may contribute to corticotropic tumor formation. In this study, 16 corticotropic adenomas of dogs with pituitary-dependent hyperadrenocorticism were screened for mutations in the Gs alpha, H-, K-, N-ras genes and the coding region of the DNA-binding domain of the glucocorticoid receptor. The cDNA fragment of the Gs alpha gene encompassed codons 159-240. The K-, and N-ras fragments spanned codons 1-71. The H-ras gene was only screened for mutations in codons 12/13 by direct sequencing of the PCR product. The cDNA fragment of the DNA-binding domain of the glucocorticoid receptor encompassed codons 410-500. The Gs alpha, K-ras, N-ras genes and the DNA-binding domain of the glucocorticoid receptor were screened by single-strand conformation polymorphism analysis. No mutations were found in the Gs alpha gene, the ras genes and the DNA-binding domain of the glucocorticoid receptor. It is concluded that mutations in the Gs alpha gene (codons 159-240), the K- and N-ras genes (codons 1-71), the H-ras gene (codons 12/13) and mutations in the DNA-binding domain of the glucocorticoid receptor do not play a role in the tumorigenesis of canine corticotropic adenomas.