This paper reviews a method for the design of peptides and proteins of predefined structure and function and provides an example. Specifically, an analog of rat growth hormone-releasing hormone (GHRH) (residues 1-23) was synthesized by solid-phase methods based on a reversed sequence of the mRNA for GHRH (1-23). The new peptide, designated GHRH 3′-5′, had a hydropathic profile similar to that of native GHRH 5′-3′ (GHRH) but had only 17% primary sequence homology. GHRH 3′-5′ specifically bound to the GHRH receptor on rat pituitary cells and to polyclonal anti-GHRH antibody in ELISA and RIA procedures. Additionally, GHRH 3′-5′ blocked the in vitro stimulation of GH RNA synthesis and in vitro and in vivo GH release mediated by GHRH. These data show that 3′-5′ GHRH with little sequence homology to native rat GHRH is an antagonist and further supports the importance of the linear pattern of hydropathy to the gross secondary and/or tertiary structure and rudimentary function of peptides and proteins. The impact of these findings on the interaction of complementary peptides is discussed.