Brain tissue of Alzheimer's disease patients invariably contains deposits of insoluble, fibrillar aggregates of peptide fragments of the amyloid precursor protein (APP), typically 40 or 42 residues in length and referred to as A beta(40) and A beta(42). However, it remains unclear whether these fibrils or oligomers constitute the toxic species. Depending on sample conditions, oligomers can form in a few seconds or less. These oligomers are invisible to solution NMR spectroscopy, but they can be rapidly (<1 s) resolubilized and converted to their NMR-visible monomeric constituents by raising the hydrostatic pressure to a few kbar. Hence, utilizing pressure-jump NMR, the oligomeric state can be studied at residue specific resolution by monitoring its signals in the monomeric state. Oligomeric states of A beta(40) exhibit a high degree of order, reflected by slow longitudinal N-15 relaxation (T-1 > 5 s) for residues 18-21 and 31-34, whereas the N-terminal 10 residues relax much faster (T-1 <= 1.5 s), indicative of extensive internal motions. Transverse relaxation rates rapidly increase to ca. 1000 s(-1) after the oligomerization is initiated.