Human GH (hGH) in the circulation of acromegalic patients and pharmacologically stimulated normal subjects consists of several monomeric and oligomeric molecular forms. However, little is known about the nature of plasma hGH under physiological conditions. We examined the molecular composition of plasma hGH secreted in response to synthetic human pancreatic tumor GRF-(1-40) (hpGRF-40), a peptide closely resembling or identical to hypothalamic GRF. The peptide (10 micrograms/kg) was injected iv into six normal men, and blood was obtained 30 min later. Plasma hGH was characterized by gel filtration and by polyacrylamide gel electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and isoelectric focusing after extraction from plasma by immunoadsorbent chromatography. At least 53% of hGH eluted as little (monomeric) hGH, 27% as big (dimeric) hGH, and 20% or less as big-big (oligomeric and spurious) hGH during gel filtration. Among the monomeric forms, the 22,000-dalton form was predominant (83%), with smaller quantities of the 20,000-dalton variant (11%), and one or more unidentified acidic forms (N alpha-acetylated, deamidated, or cleaved hGH) (6%) also present. The molecular composition of plasma hGH secreted in response to hpGRF-40 is similar to that released after pharmacological stimuli or that circulating in acromegaly.