In normal young adults, N400 amplitude varies inversely with the extent to which a word has been primed by its preceding semantic context. Based on a series of behavioral studies, it appears that in Probable Alzheimer's patients (PAD) the organization of semantic memory is disrupted such that specific items within a category lose their distinction, although superordinate information remains relatively intact. The present study examined whether the N400 gradient which has been found with normal young adults would also reflect this loss of discriminability among semantically related items in PAD patients. Ten normal young adults, 10 normal elderly, and 6 "mild" PAD patients made speeded (but accurate) sense/nonsense decisions to the terminal words of a series of highly constrained sentence contexts. The terminal words belonged to one of four stimulus types which varied as a function of relatedness to a highly expected word. Counter to our predictions, N400 amplitude was identically responsive to semantic relatedness in the young normal and PAD groups, but was characterized differently in the normal elderly. Given the significantly greater number of errors committed by PAD patients, we concluded that their disruption in semantic processing occurs at some point between the elicitation of N400 and the generation of the reaction time response. The anomalous N400 pattern in the normal elderly appeared to be strategy related and superimposed upon an otherwise normal semantic network.