Recombinant cDNA clones containing the repetitive sequences of major retroposon families--ID, B2 and L1--were isolated from the cDNA bank of rat brain. Their organization was studied by sequencing and hybridization analysis. It was shown for L1 that: 1) its transcripts are mainly found in the fraction of poly(A) +mRNA; 2) there is a population of full-length elements present in the rat genome. We also tried to estimate the copy number of these elements. Some aspects of their organization are discussed.