The regulation of elastin production has been evaluated and compared in two systems in the developing sheep: nuchal ligament and lung. Absolute rates of production in explant culture were estimated by immunoprecipitation of newly synthesized elastin and determination of precursor pool specific activities. Lung elastin production increased about 2.5-fold while nuchal ligament production increased about 9-fold during the latter half of gestation. In comparison, the activities of elastin messenger RNA(mRNAE) of these tissues were determined in a cell-free system by immunoprecipitation. The increases in mRNAE activity largely paralleled the rises in elastin production, suggesting that elastin synthesis in these tissues is regulated, at least in part, by the availability of translationally active mRNAE.