Human growth hormone (hGH) concentrations in plasma often fall to levels not detectable by RIA. These so-called basal levels prevail during the greater part of the day. Since hGH is involved in the homeostasis of several metabolic processes, it is important to examine its exact plasma concentration and secretory pattern during basal periods. We used an immunoadsorbent technique to extract hGH from large plasma samples to precisely measure basal hGH concentrations and their variation with time. Blood samples (20 mL) were drawn from 12 normal subjects in the fasted and rested state every 15 minutes over a three-hour period. Plasma hGH levels varied over three orders of magnitude (range, 34 to 60,000 pg/mL). During basal periods, episodes of secretory pulses, of moderate sustained secretion, and of complete secretory inactivity occurred. Women had significantly higher overall hGH levels as well as basal hGH levels than men, but no significant sex difference in the pulse frequency during basal periods could be detected in the limited time allotted for study. No convincing relationship was noted between variations in plasma glucose and the secretory pattern of hGH, or vice versa. We conclude that hGH is secreted in an episodic fashion during basal periods. Conceptually, basal and stimulated hGH secretion may be viewed as extremes of a continuous spectrum of pituitary activity, basal hGH levels are lower than heretofore appreciated, the known tendency of women to higher hGH levels is also evident in the basal range, and oscillations in plasma glucose do not affect the microsecretory pattern of hGH, nor are endogenous hGH pulses followed by acute changes in glycemia.