Many chromaffin cell tumors contain somatostatin (SS), and most of them are receptor-positive by in vitro autoradiography and by in vivo administration of radiolabeled SS analogs. We evaluated the effect of a 2-hour infusion of 50 µg octreotide on plasma norepinephrine (NE) and epinephrine (E) levels in 6 patients with chromaffin cell tumors. To ascertain the biological activity of octreotide, plasma insulin levels were also measured. Infusion of octreotide was followed in all the patients but 1 by a progressive decrease of plasma NE levels. On the average, plasma NE decreased significantly during octreotide administration, halving its baseline levels at the end of the infusion (51.3 ± 11.46%, p < 0.05) and rising again slowly thereafter. A slight reduction of plasma E levels was also recorded at the end of octreotide infusion (76.1 ± 13.77% of baseline, NS) with a prompt return of hormone concentrations to preinfusion values. During octreotide administration, plasma insulin displayed an early and steep fall (49.7 ± 4.61 % of baseline, p < 0.03, at 60 min) with a tendency to escape from inhibition before termination of the infusion. In 5/6 patients, 2 of whom were normotensive before the study and 1 the day of the test, blood pressure did not change during octreotide infusion. In the other patient, blood pressure fell from baseline values of 160/100 to 120/ 70 mm Hg at 120 min and rebounded to 205/100 mm Hg at 240 min. In conclusion, the short-term administration of low-dose octreotide is capable of lowering NE levels, though with no consistent effect on blood pressure, in patients with chromaffin cell tumors. The effect of sustained treatment with octreotide seems to be worth investigating.