We synthesized the N-terminal hexapeptide fragment of IGF II to study potential binding to NMDA receptors in analogy to the N-terminal tripeptide of IGF I. The amino acid sequence of the hexapeptide is furthermore identical with the C-terminal sequence of the casiragua insulin B chain. The hexapeptide did not bind to the NMDA receptors, but was found to promote [3H]-thymidine incorporation into fibroblasts at concentrations of 10(-8) - 10(-5) M in a dose-dependent manner. Since [125I]-hexapeptide did not bind to IGF receptors, indirect competition studies using either labelled IGFs or insulin had to be used. The competition of hexapeptide at a concentration of 10(-5) M with labelled IGF I or II was about equal to that of 10(-9) M IGF I or II. IGF receptors were apparently up-regulated by the hexapeptide, as has also been described for insulin. When using casiragua insulin as labelled ligand, IGF II and casiragua insulin competed with equal potency, whereas the hexapeptide at 10(-7) M caused an apparent up-regulation of the casiragua insulin binding sites. Our results that the hexapeptide stimulates [3H]-thymidine incorporation and up-regulates IGF II and casiragua insulin binding sites may be connected to one or several of the following findings: the hystricomorph insulins--of which the casiragua insulin is a member--stimulate DNA synthesis to a greater extent than other insulins; the insulin and type 1 IGF receptor binding regions are localized predominantly in the C-terminal region of the insulin B chain; and the "cooperative" site regulating the affinity of the insulin receptor is also located in the C-terminal region of the insulin B chain. Further experiments will be needed to clarify the exact mechanism.