Thyroid disorders affect renal function, which involves changes in local renin angiotensin system (RAS). Angiotensin peptide levels in the tissue are regulated by the activity of several aminopeptidases (AP) known as angiotensinases. The nature and consequences of the thyroid-induced RAS changes are not completely understood. We investigated the relationship between thyroid status (hyper- and hypothyroidism) and several kidney AP actions involved in RAS control. We have determined fluorometrically soluble (SOL) and membrane-bound (M-B) alanylaminopeptidase (AlaAP), glutamylaminopeptidase (GluAP) and aspartylaminopeptidase (AspAP) activity using naphthylamide derivatives as substrates. Sprague-Dawley rats were divided into three groups--control, hyperthyroid, and hypothyroid. Hyperthyroidism was induced by daily subcutaneous injection of L-thyroxin (300 microg/kg/day). Hypothyroidism was induced by continuous administration of methimazole (0.03%) in drinking water. Hypothyroid animals demonstrated a significant increase in SOL and M-B GluAP activity in renal cortex and a decrease in M-B AlaAP compared to euthyroid rats. This result may suggest higher Ang III availability. In hyperthyroid animals, M-B AlaAP and M-B AspAP activity increased significantly, which may suggest increased Ang III to Ang IV metabolism and greater formation of Ang 2-10, respectively. In contrast, no differences were observed between euthyroid and hypothyroid animals for SOL and M-B AP activity in renal medulla. However, hyperthyroid animals demonstrated a significant decrease in SOL and M-B GluAP activity compared to euthyroid rats, which may suggest a greater availability of Ang II in renal medulla. Alterations in angiotensin metabolism may, in part, account for some changes in renal function during thyroid disorders.