The secondary structures in solution of the synthetic, naturally occurring, amyloid beta peptides, residues 1 to 42 [beta (1-42)] and beta (1-39), and related fragments, beta (1-28) and beta (29-42), have been studied by circular dichroism and two-dimensional nuclear magnetic resonance spectroscopy. In patients with Alzheimer's disease, extracellular amyloid plaque core is primarily composed of beta (1-42), whereas cerebrovascular amyloid contains the more soluble beta (1-39). In aqueous trifluoroethanol solution, the beta (1-28), beta (1-39), and beta (1-42) peptides adopt monomeric alpha-helical structures at both low and high pH, whereas at intermediate pH (4 to 7) an oligomeric beta structure (the probable structure in plaques) predominates. Thus, beta peptide is not by itself an insoluble protein (as originally thought), and localized or normal age-related alterations of pH may be necessary for the self-assembly and deposition of beta peptide. The hydrophobic carboxyl-terminal segment, beta(29-42), exists exclusively as an oligomeric beta sheet in solution, regardless of differences in solvent, pH, or temperature, suggesting that this segment directs the folding of the complete beta (1-42) peptide to produce the beta-pleated sheet found in amyloid plaques.