Tachycardia during reversal of one-kidney, one-clip hypertension in conscious rats. The efferent sympathetic activity of the heart was investigated during reversal of one-kidney, one-clip (1K,1C) hypertension in conscious, freely moving rats using changes in heart rate as an index. One-kidney, one-clip hypertensive rats presented marked tachycardia during the first 8 h after unclipping. Mean arterial pressure fell 28% 3 h after unclipping and 37% 8 h after unclipping. In a separate experiment, short term infusion (15 min) of sodium nitroprusside in conscious freely moving rats produced similar tachycardia in hypertensive and sham-operated rats. The present results indicate that the baroreceptor reflex of the heart was operating normally during the prompt reversal of hypertension after unclipping. 