In order to examine the role of central catecholaminergic neurons in hypertension, the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines, was studied in the hypothalamus, midbrain, pons-upper medulla, mid-medulla and lower medulla of one-kidney, one clip (1-K,1C) and two-kidney, one clip (2-K,1C) hypertensive rabbits and their respective operated controls (1-K,1 Cc and 2-K,1 Cc). Comparing the 1-K,1 C group to the 1-K, 1 Cc group, the activity of TH was increased by 79% in the hypothalamus (P less than 0.02), 37% in the mid-medulla region (P less than 0.02) and was unchanged in the midbrain, pons-upper medulla and the lower medulla. Comparing the 2-K,1 C group to the 2-K,1 Cc group, the activity of TH was increased by 89% in the mid-medulla (P less than 0.01), decreased by 36% in the pons-upper medulla (P less than 0.01) and unchanged in the hypothalamus, midbrain and lower medulla. These results indicate that similarities and differences exist in the contribution of central catecholaminergic neurons to the pathophysiology of 1-K,1 C and 2-K,1 C hypertension in rabbits.