An interesting pattern in the genetic code was recently observed: Codons for hydrophilic and hydrophobic amino acids on one strand of nucleic acid are complemented by codons for hydrophobic and hydrophilic amino acids on the other strand, respectively. The average tendency of codons for 'uncharged' (slightly hydrophilic) amino acids is to be complemented by codons for 'uncharged' (slightly hydrophilic) amino acids. We have postulated that this pattern can result in the binding of peptides that are encoded by complementary RNA strands and we have presented supporting evidence. In this report we demonstrate the specific and high-affinity binding of naturally occurring peptides [corticotropin (ACTH) and γ-endorphin] to synthetically derived counterparts that were specified by RNA sequences complementary to the mRNA for ACTH and γ-endorphin, respectively. That this binding might result from one peptide being an 'internal image' of the other was strongly suggested by the observation that antibody to the peptide that was encoded by the complementary RNA for ACTH recognized the adrenal cell ACTH receptor. Based on these findings, a theory on the evolution of peptides and their receptors is suggested.