Various synthetic fragments of the amino terminal sequence of human growth hormone (HGH) have been studied with respect to their in vitro action on glucose uptake and in vivo insulin potentiating effects. The results show that the biologic activities so far investigated are confined to sequences containing the octapeptide, Leu, Ser, Arg, Leu, Phe, Asp, Asn, Ala. There is also a correlation between the in vitro reversal of somantin induced inhibition of glyceraldehyde 3 phosphate dehydrogenase by the active peptides and their insulin potentiating effect in vivo. The GAPD system provides a convenient and rapid means for monitoring such peptides, but its physiologic significance is at present in doubt. Large doses (10 mg/kg weight) of the active peptide HGH 1-15 do not increase the degree of lowering of blood glucose, but prolong the effect of the peptide and depress plasma immunoreactive insulin (IRI) in rabbits. The mechanism of action of the active sequence has not been established at present, and the physiological significance is not yet known.