During our initial experience with endoscopic third ventriculostomies, we observed intraoperative bradycardia and postoperative hyperkalemia. The present study was carried out to verify the consistency of these initial observations. Intraoperative heart rate (HR) changes and postoperative serum K + changes were studied prospectively in 20 patients of endoscopic third ventriculostomy. Another 6 patients who underwent endoscopic procedures other than ventriculostomy acted as controls. The anaesthetic technique and intraoperative and postoperative fluid regimen were similar in all patients. Serum K + concentrations were measured intraoperatively and once a day for the next 5 days. The third ventriculostomy group exhibited a significant slowing of the heart rate during the fenestration of the floor of the third ventricle (112 +/- 26 to 101 +/- 28 bpm, p < 0.001) and also at the time of the reversal of the neuromuscular block at the end of surgery (104 +/- 29 to 96 +/- 33 bpm, p < 0.01). The control group did not exhibit similar changes in the heart rate. The postoperative increase in serum K + values in the ventriculostomy group (0.82 +/- 0.55 mmol/L) was higher than that in the control group (0.10 +/- 0.44 mmol/L) (p < 0.01). Endoscopic third ventriculostomy is associated with a significant bradycardia at the time of fenestration and at the time of reversal of the neuromuscular block. The procedure is also associated with a postoperative increase in serum K + values. We propose a mechanism involving distortion of the posterior hypothalamus, which accounts for the bradycardia and postoperative hyperkalemia.