Reassociation kinetics of macronuclear (Ma) DNA of the ciliate Tetrahymena pyriformis GL has been studied. The genome size calculated by the kinetic complexity of DNA constitutes of 2.0.10(8) base pairs, which corresponds to the molecular weight of 1.2.10(11) dalton. About 90% of the Ma DNA fragments of 200-300 base pairs in length reassociate at a rate corresponding to the unique sequences and 3--5% of the nucleotide sequences at a ten times exceeding rate. About 3--5% of the genome consists of sequences reassociating at Cot practically equal to zero. Most of the zero time reassociating DNA seems to be represented by inverted nucleotide sequences. This conclusion was drawn from the study of the at zero time reassociating DNA isolated preparatively by hydroxyapatite chromatography. This fraction shows an approximately 75% resistance to S1-nuclease treatment which is independent from the original DNA concentration. Heat denaturation and reassociation are mutually reversible yielding hyper- and hypochromic effects. The majority of the inverted sequences are likely to be unique, about 20% is repeated scores of time in the Tetrahymena genome. This assumption has been made on the strength of reassociation of the zero time [125I]DNA fraction (after breaks in single-strand hair pin loop with the aid of S1-nuclease) with non-fractionated Ma DNA. According to the equilibrium distribution in a CsCl density gradient the mean nucleotide content of inverted [14C]DNA sequences does not differ from that of non-fractionated DNA. Consequently the largest part of the isolated fraction of inverted nucleotide sequences of the Tetrahymena genome are not fragments of ribosomal genes. The evidence of reassociation kinetics of Ma DNA fragments of various lengths allows to suggest that inverted sequences from large blocks (no less than 6000 base pairs) and scores of times repeated sequences may alternate with the regions of unique sequences.