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Changes in the properties and distribution of LDH isoenzymes have been studied in the tissues and fluids of the reproductive tract during the oestrous cycle and during postnatal development in the rat. The specific activity values of the tissue LDH over this period indicate a sequential response along the length of the tract. In the extracellular secretions of the tract, peak activities coincide with, or closely follow, peak values in adjacent tissues and isoenzyme patterns tend to bear a reciprocal relationship between individual sections of the tract tissues and the enclosed secretions. The high levels of extracellular activity occurring within the oviducal secretion appear to be the result of contributions from the circumjacent tissues along the length of the tract, the Fallopian tube being indicated as an especially rich source. Hormonal and developmental influences on the synthesis of LDH isoenzymes are discussed in relation to the changes observed.