We have measured the reassociation kinetics of DNA from the micronucleus and from the macronucleus of the hypotrichous ciliate Oxytricha. The micronuclear DNA reassociates with at least a two-component reaction, indicating the presence of both repeated and non-repeated sequences. The kinetic complexity of micronuclear non-repeated DNA is in the range of 2 to 15 X 10(11) daltons; the haploid DNA content of the micronucleus is 4 X 10(11) daltons (0.66 pg), measured microspectrophotometrically. The DNA of the macronucleus reassociates as a single second-order reaction, with a kinetic complexity of 3.6 X 10(10) daltons. A comparison of the kinetic complexities of micronuclear and macronuclear DNAs suggest a 5 to 30 fold reduction in DNA sequence complexity during the formation of a macronucleus from a micronucleus. Macronuclear DNA is in pieces with an average molecular weight of 2.1 X 10(6) daltons. Since the kinetic complexity of macronuclear DNA is 3.6 X 10(10) daltons, the macronucleus must contain about 17,000 different kinds of DNA pieces. Each macronucleus contains 3.5 X 10(13) daltons (58 pg) of DNA, indicating that each sequence must be present about 1000 times per macronucleus or 2000 times per cell.