Alzheimer's disease is characterized by filamentous depositions of amyloid A4 protein in the brain. The first precursor of A4 protein that has been described consists of 695 amino acids (PreA4(695)). Until now, three types of amyloid precursor mRNAs (PreA4(770), PreA4(751) and PreA4(695)), produced by alternative splicing, have been detected. We analysed the differential expression of these mRNAs in various rat tissues by PCR and show that (1) there exists a fourth type of mRNA, PreA4(714); (2) in all tissues except the brain the PreA4(695) mRNA is less abundant than the other types of mRNAs; in the brain, however, the PreA4(695) mRNA predominates by far. The same observations hold true for human tissues. The possible function of this differential splicing is discussed.