The physiological mechanisms by which growth hormone (somatotropin) exerts its several metabolic activities remain poorly understood. In particular, there is disagreement as to whether the diabetogenic and lipolytic activities of the hormone are intrinsic properties of the molecule or are the result of contamination with other pituitary components. The availability of recombinant-DNA-derived bovine growth hormone (rebGH) presented an opportunity to compare the biological activities of rebGH and pituitary bGH in the absence of pituitary contaminants. Pituitary bGH and rebGH were immunologically identical in the radioimmunoassay for bGH, and good agreement was obtained for the potency of the latter measured by radioimmunoassay (1.6 units/mg) and the dwarf-mouse bioassay (1.4 units/mg). The lipolytic activity in vitro was examined by measuring the release of glycerol from rat epididymal fat maintained in the presence of dexamethasone (0.2 microgram/ml) and the material to be tested (0.1 and 0.2 mg/ml). Although two preparations of pituitary bGH stimulated a significant (P less than 0.01) increase in glycerol production, neither rebGH nor recombinant-DNA-derived chicken GH was lipolytic. However, when rebGH was intravenously injected into three sheep (0.15 mg/kg), the increase in plasma nonesterified fatty acids was similar to that measured with the same dose of pituitary bGH. Insulin-tolerance tests were conducted in sheep before and after treatment with rebGH and pituitary bGH (four subcutaneous injections of 0.15 mg/kg). Although the effect of rebGH was less than that of the pituitary hormone, both significantly impaired the ability of insulin to lower the concentration of plasma glucose. These data suggest that the lipolytic and diabetogenic activities of bGH are intrinsic properties of the molecule. However, the lipolytic activity may only become apparent after either modification of the molecule in vivo or activation of a lipolytic intermediate.