C-type natriuretic peptide (CNP) is a recently identified member of the family of natriuretic peptides that plays an important role in the regulation of blood pressure, renal function and volume homeostasis. Its effects are mainly hypotensive and natriuretic. CNP is also considered to be an autocrine/paracrine regulator of the endothelium as well as a neuropeptide. Little is known about its role as a circulating substance. There are only few data of CNP plasma levels and changes in disease. Using a highly specific radioimmunoassay we established for the first time CNP plasma levels of normal pregnancies 5.9 +/- 1.5-8.6 +/- 2.1 pg/ml) and a non-pregnant control group (8.1 +/- 1.7 pg/ml). In contrast to atrial natriuretic peptide and brain natriuretic peptide we have not found significant differences of the plasma levels during normal pregnancy and no difference to the non-pregnant control group.