1. In these studies the role of the sympathetic nervous system (SNS) has been investigated in two models of renovascular hypertension in the rat and after surgical reversal. 2. Plasma noradrenaline (NA), heart rate and blood pressure (BP) were measured simultaneously in conscious rats with one-kidney, one-clip (1K1C) (4-6 weeks) and two-kidney, one-clip (2K1C) (4-6 and >16 weeks) hypertension, and parallel loose clip controls, before and 48 hours after unclipping. BP in all hypertensive groups fell to normal after unclipping. 3. Plasma NA was elevated in 1K1C hypertension and fell on unclipping. Conversely, in early 2K1C hypertension plasma NA was unaltered before and rose after unclipping. Plasma NA did not change with unclipping in chronic 2K1C hypertension and was not different from controls. Heart rate showed a similar pattern. Unclipping loose clip control rats produced no change in BP, plasma NA or heart rate. 4. Heart rate was correlated with plasma NA in 1K1C hypertension, and changes in these variables on unclipping were correlated in all three models. BP was only correlated with plasma hypertensive rats. 5. In contrast, renal and cardiac NA levels showed a remarkably similar pattern in 1K1C and 2K1C hypertension of 4-6 weeks duration. Ipsilateral renal NA was reduced in loose clip rats compared to sham-operated controls. This is most likely due to renal denervation during clipping. Hypertension produced a further reduction in ipsilateral renal NA and a reduction in cardiac and contralateral renal (2K1C) NA. 6. It is concluded that the SNS may have a minor role in 1K1C, but not in 2K1C, renovascular hypertension. However, changes in SNS activity upon reversal of hypertension do not explain the BP fall in either model. Changes in renal and cardiac levels of NA in renovascular hypertension are primarily secondary to sustained elevation of BP.