The effect of unilateral sympathetic denervation and reinnervation of the lung on a variety of circulatory parameters was investigated in urethane-anaesthetized rats. The left main stem bronchus together with its surrounding nerves was cut and reanastomosed in 40 rats: 12 intact rats served as controls. Final experiments were performed after 0 days to 10 months: the left stellate ganglion was stimulated. The effect was greatest at 20 Hz. Pulmonary arterial pressure increased significantly (P < 0.05) by 10% and pulmonary flow decreased significantly (P < 0.05) by 16% in the control rats; no effect on these parameters was found in acutely denervated rats. The stimulus-elicited change in pulmonary arterial pressure reappeared 1 week after unilateral hilar stripping and gradually returned during reinnervation. After 10 months, the increase in pressure was significantly (P < 0.05) larger than that of the control by 50%, whereas the noradrenaline content of the reinnervated lung was significantly (P < 0.05) smaller than that of the intact side by 48%. This discrepancy may reflect denervation hypersensitivity of vascular muscle cells. The operations had no effect on systemic circulation: heart rate and systemic arterial pressure increased and aortic blood flow decreased to the same extent in all experimental groups during nerve stimulation. These results suggest that sympathetic reinnervation of the rat lung starts within 1 week but that reinnervation is still incomplete 10 months after unilateral hilar stripping.