1. Plasma noradrenaline (NA), blood pressure (BP) and heart rate (HR) were measured simultaneously in conscious rats under basal conditions in the early phase (4–6 weeks) of one-kidney, one-clip hypertension (1K1C), in the early (4–6 weeks) and chronic (> 16 weeks) phases of the two-kidney, one-clip model (2K1C) and in age-matched loose clip control animals before and 2 days after unclipping. 2. The elevated BP in all three hypertensive groups fell to normal after unclipping, whereas removal of the constricting clip in loose clip controls had no effect on BP. 3. Plasma NA was elevated in 1K1C hypertension ( P < 0.05) and fell slightly but non-significantly on unclipping. However, in the early phase of 2K1C hypertension plasma NA was unaltered before and rose significantly ( P < 0.05) after unclipping. Plasma NA did not change with unclipping in the chronic phase of 2K1C hypertension and was not different from controls. Unclipping loose clip control animals produced no change in plasma NA. 4. Changes in HR on unclipping followed a similar pattern to changes in plasma NA: changes in the two variables were significantly correlated in all three models (1K1C: r = 0.61, P < 0.005; early 2K1C: r = 0.45, P < 0.05; chronic 2K1C: r = 0.62, P < 0.01). However, BP was only correlated with plasma NA in 1K1C hypertension ( r = 0.49, P < 0.02) and not in either phase of the 2K1C model. There was also a highly significant correlation between HR and plasma NA in 1K1C hypertension ( r = 0.71, P < 0.001). The pattern of the changes in plasma NA and HR that occurred with reversal of 1K1C hypertension was significantly different from those in the early phase 2K1C model ( P < 0.05). 5. These data suggest that there is sympathetic nervous system (SNS) activation in the early phase of 1K1C hypertension, but provide no evidence for increased sympathetic activity in either the early or chronic phases of the 2K1C model. Neither do they support the hypothesis that the fall in BP with unclipping in this model is mediated by reduced SNS activity.