The conformation-dependent antibodies Tau-66 and Alz-50 recognize discontinuous epitopes on the tau molecule (residues 155-244 & 305-314 and 5-15 & 312-322, respectively), thereby defining two distinct conformations. In double- and triple-label immunofluorescence experiments we discovered that specific populations of neurofibrillary tangles display either the Alz-50 or the Tau-66 conformation, but not both. In combination with other antibodies to several domains of the molecule we demonstrate that the conformation recognized by Alz-50 seems to be an early event in the formation of neurofibrillary tangles. This conformation is characterized by the presence of predominantly intact N- and C- termini. By contrast, the Tau-66 conformation is likely a later event in tangle development, being favored in structures containing truncations of both the N- and C- termini. We propose a sequence of events that occurs during the formation and evolution of neurofibrillary tangles based on the initial conformation adopted by tau. In this scheme, the Tau-66 conformation in neurofibrillary tangles may arise from amino and carboxy truncation of tau after it has assumed the Alz-50 conformation. These results indicate that tau structure within the NFT is dynamic in that tau can undergo a "refolding" event following N- and C-terminal truncation.