The pharmacokinetics and acute effects of an authentic recombinant DNA-derived human growth hormone (rhGH) produced by genetically engineered mammalian cells were determined in 12 healthy volunteers following intravenous (i.v.), intramuscular (i.m.) and subcutaneous (s.c.) administration of 4IU (1.3 mg) hGH/m<sup>2</sup> body surface area. Following i.v. administration, apparent elimination half-life of rhGH was 18 min. Following i.m. administration, a mean peak serum concentration of 36.9 ng/ml (range 13-61 ng/ml) occurred at 3 h, and following s.c. administration, more sustained but lower serum concentrations occurred, with mean peak concentrations of 16.4 and 16.3 ng/ml at 4 and 6 h (ranges 9.0-27.5 ng/ml and 6.5-35.5 ng/ml at 4 and 6 h, respectively). The mean area under the curves was lower after s.c. (134 ± 48 ng·h·mH) than after i.m. (194 ± 48 ng·h·ml<sup>-1</sup>) injections (p < 0.03). Comparable results were obtained for the same dose of rhGH given subcutaneously in concentrations of either 4 IU/ml or 10 IU/ml. Both i.m. and s.c. administrations caused similar increases in free fatty acids at 4 h and insulin-like growth factor I at 24 h. Insulin, C-peptide and blood glucose were almost unchanged during the first 4 h after administration, whereas leukocytes increased significantly (p < 0.0001). Local and systemic tolerance were good, and no adverse reactions were observed. In a GH-deficient child, hGH serum levels between 10 and 20 ng/ml were demonstrated for a period of 8 h after s.c. administration of 0.07 IU rhGH/kg body weight.