In 3 acromegalic patients plasma growth hormone (GH) concentrations fluctuated widely when measured frequently throughout the day and night. In 2 of these patients occasional plasma GH concentrations were within the normal range. The expected normal rise of plasma GH with the onset of sleep did not occur. Significant rises in plasma GH occurred on at least one occasion to the following provocative stimuli: hypoglycemia, hyperglycemia and arginine infusion. The variability of plasma GH concentration is consistent with persistent hypothalamic regulation, which had not been suppressed by high plasma GH concentrations.