The time course and defining factors involved in the formation of neurofibrillary tangles (NFT) and senile plaques (SP) in Alzheimer’s disease (AD) brain are unknown. Above ground nuclear weapons testing in the late 1950’s and early 1960’s led to significantly increased levels of 14C in the atmosphere and the carbon cycle. Because SP and NFT, once formed, are relatively resistant to degradation, the 14C levels observed in SP and NFT should reflect their year of formation. Through the use of accelerator mass spectrometry (AMS) and the measurement of 14C levels, we have determined the average age of formation of isolated SP and NFT fractions in bulk brain samples of 6 AD subjects. While a heterogeneous process of formation is indicated, we find that NFT pre-date SP in the majority of cases. In addition, it is clear that these features, once formed, have a much slower carbon turnover rate than normal brain tissue and are not in a formation/enzymatic degradation equilibrium.