In this study, the pharmacokinetics, efficacy, and tolerability of 25 and 100 micrograms of octreotide given t.i.d. for 7 days subcutaneously were investigated in 12 healthy male subjects. Serum concentrations of the drug were well reproducible within 1 wk. Octreotide significantly raised 24-h median intragastric pH on day 1, but no longer on day 6. Peptone-stimulated gastric acid and volume secretion were markedly less suppressed by octreotide on day 7 compared with day 2. Peptone-stimulated gastrin release was abolished on days 2 and 7, as was peptone-stimulated insulin release. Blood glucose was altered in a biphasic pattern on days 2 and 7. All effects of octreotide were without clear-cut dose-response relationship. A mean half-life of 115 min was calculated. Dose-unrelated side effects (e.g., abdominal cramps, diarrhea, and fatty stools) were registered. In conclusion, octreotide is a powerful inhibitor of gastric acid and volume secretion during acute treatment. Its loss of efficacy during a 1-wk administration may be due to the adaptation of somatostatin receptors and hormonal counterregulation.