Current evidence indicates that resistance to insulin due to nocturnal secretion of growth hormone plays an important role in the Dawn phenomenon and that day-to-day variability in growth hormone secretion makes this condition difficult to manage. We therefore assessed the effect of a long-acting somatostatin analog (L363,586) on overnight plasma glucose and growth hormone levels in six patients with insulin-dependent diabetes mellitus. The analog (600 micrograms) was administered intranasally at bedtime to determine whether the inconvenience of an additional injection could be avoided. Compared to control experiments, in which saline was administered intranasally, overnight increases in plasma glucose concentrations were reduced in all subjects by nearly 70% (48 +/- 19 v 148 +/- 26 mg/dL, P less than .01), plasma growth hormone was maintained at basal levels throughout the night (less than 2 v 8 to 12 ng/mL, P less than .01), and the analog was well tolerated. We conclude that pharmacologic blockade of growth hormone secretion may be a helpful approach to management of the Dawn phenomenon when it cannot be done safely and effectively by adjusting insulin doses.