Transient postocclusive renal vasospasm occurred in nine conscious postoperative monkeys with electromagnetic flowmeters implanted on the left renal artery. During an interval of 5-7 days, beginning approximately 1 wk after the implantation of an electromagnetic flowmeter and pneumatic occlusive cuff on the renal artery, occlusions produced by 2- to 15-s inflations of the cuff were invariably followed by flow reduction of 40%-90%, which began 30-60 s after occlusion and lasted from 3 to 60 min. These flow reductions were not related to changes in heart rate, cardiac output, blood pressure, nor to flow in other branches of the aorta; nor were they affected by papaverine (5 mg/kg), propranolol (1 mg/kg), phentolamine (3-5 mg/kg per min), or dipyridamole (1 mg/kg). This response to renal artery occlusion disappeared after 5-7 days and did not recur. This phenomenon should be kept in mind when planning experiments involving long-term implantation of flowmeters on the renal arteries of monkeys.