The dipteran salivary gland is considered from several points of view. Its development in relation to chromosomal puffing suggests that this tissue, perhaps more than any other, represents major evidence for differential activation and inactivation of genes during development. From studies of this tissue we have gained further insight into concepts of tissue and stage specificity. Factors regulating puffing during development include the natural hormone, ecdysone, as well as a host of other substances and environmental factors which affect cellular metabolism. Breakdown of the salivary gland seems ultimately to be controlled by chromsomal puffing. In this case one of the first detectable biochemical alterations in the gland is the accumulation of the enzyme DNase. This enzyme may play a role in digesting the chromosomes themselves, thus accomplishing regulated self-destruction of the cell's synthetic machinery.