The induction of somatomedin C (SM-C) was evaluated in 30 hypopituitary patients and in normal subjects using a sensitive, specific RIA for SM-C. Two hypopituitary patients each received an iv bolus of 0.1 U human GH (hGH)⁄kg BW, followed by continuous infusion of 0.01 U hGH⁄kg-h for 12 h. The earliest rise in SM-C was detectable between 6-8 h. In response to the im administration of hGH, hypopituitary patients demonstrated a delay of 8 h before the rise in SM-C, a peak response between 16-28 h, and a fall nearly to basal levels by 48 h. The delay in the serum SM-C response to the administration of hGH suggests that SM is not stored in a readily releasable form and that the effect of GH on SM production may be through the stimulation of de novo synthesis. Graded doses of hGH in a sequence of 0.1, 0.2, 0.4, and 0.8 U/kg (im) elicited a stairstep pattern of SM-C response. When the sequence of administration of these doses was altered, however, it was apparent that the magnitude of the SM-C response was determined less by the dose of hGH employed than by the prior experience of the patient with hGH. When two identical doses of hGH were administered at 48-h intervals, the response to the second dose was uniformly greater than that to the first. Two of 28 hypopituitary children receiving hGH therapy failed to raise their SM-C levels out of the hypopituitary range and 6 other responses were subnormal. These noninducers were undernourished and had normal growth rates in spite of persistently low SM-C levels. Sephadex G-200 chromatography of the serum from 2 of these patients revealed neither an elevation in [125Ι]SM-C binding in the large 150,000 (150 K) mol wt region nor a significant increase in endogenous SM-C activity after hGH. This contrasts with the pattern seen in most hypopituitary patients in whom hGH causes a marked increase in endogenous activity in the 150 K region as well as the emergence of [125I]- SM-C binding in this region. From these results, it is concluded that the 150 K SM-binding complex is GH dependent and is a major determinant of the immunoreactive SM-C measured in native serum.