The expression of matrix metalloproteinase of the first type was studied in frontal sections of the adult rat brain one month after a single intracerebroventricular injection of P-amyloid peptide (25-35), which is known to be a well-known model of the development of Alzheimer's disease. Brain sections were stained immunocytochemically to detect MMP-1 expression, and histologically to reveal the state of hippocampal neurons. Administration of P-amyloid peptide induced a significant degeneration of cells in the dorsal hippocampus. This was demonstrated by a significant decrease in the total number of cells and by the appearance of acidophilic neurons of altered (often triangular) shape. Altered cells were most often found in the hippocampal field CA3, and in a smaller quantity in the CA1 field. MMP-1-like immunoreactivity was found in the same hippocampal areas, the staining being restricted to the cells of altered shape (staining of somata and primary neurites). The data suggest possible involvement of the type 1 metalloproteinase in the development of Alzheimer's disease.