Objectives - The purpose of the present study was to examine human pituitary adenomas for nitric oxide synthase (NOS) activity by immunohistochemical and enzymatic methods. Materials and methods - Adenomatous tissue from 16 patients were obtained during operation and stained immunohistochemically for hormone production and for the three NOS isoenzymes. Cell types that expressed NOS immunoreactivity (IR) were identified, and the NOS isoform was noted. NOS activity was measured enzymatically by the conversion of L-arginine to L-citrulline in tissue samples. Results - Endothelial cells of pituitary adenomas showed increase of eNOS IR compared with control tissue. The nNOS and iNOS IR were the same in adenomas and controls. There was no correlation between NOS IR and NOS activity measured enzymatically and the endocrine activity of the tumour or other clinical variables. Conclusion - The observation of increased eNOS IR in endothelial cells of adenomas may suggest that NO plays a role in the regulation of blood flow in pituitary adenomas.