The Alzheimer peptide fragment A beta(12-28) was studied at millimolar concentration by parallel experiments with high-resolution nuclear magnetic resonance (NMR) and circular dichroism (CD) in solution at a pH close to the isoelectric point of the peptide. A preparation procedure using low temperature and low ionic strength buffer gave a sample with stable and reproducible properties. Reversible changes in secondary structure and state of aggregation were studied by variation of temperature. High-temperature promotes aggregation and beta-sheet induction, whereas low-temperature shifts the equilibrium toward low molecular weight fractions and less beta-sheet like structure. NMR diffusion experiments show that the dominating, most low molecular weight fraction is monomeric. With increasing temperature, residues F(20)A(21)E(22), overlapping with the so-called central hydrophobic segment of the A beta peptide, exhibit the most pronounced alpha-proton NMR secondary chemical shift changes from random coil toward more beta-sheet like structure. High ionic strength also promotes aggregation and beta-sheet induction. The combined spectroscopic results, including also molecular weight estimations by cutoff filters, are summarized in a scheme in which monomeric mostly random coil and heterogeneous aggregated partly beta-sheet forms of the peptide are in a temperature-dependent equilibrium, a situation which corresponds to an early stage of the fibrillogenesis.