Endocrine cells in the larval midgut of Drosophila melanogaster are recognized by antisera to seven regulatory peptides: the allatostatins A, B, and C, short neuropeptide F, neuropeptide F, diuretic hormone 31, and the tachykinins. These are the same peptides that are produced by the endocrine cells of the adult midgut, except for short neuropeptide F, which is absent in adult midgut endocrine cells. The anterior larval midgut contains two types of endocrine cells. The first produces short neuropeptide F, which is also recognized by an antiserum to the receptor for the diuretic hormone leucokinin. The second type in the anterior midgut is recognized by an antiserum to diuretic hormone 31. The latter cell type is also found in the junction between the anterior and middle midgut; an additional type of endocrine cell in this region produces allatostatin B, a peptide also known as myoinhibitory peptide. Both types of endocrine cells in the junction between the anterior and middle midgut can express the homeodomain transcription factor labial. The copper cell region contains small cells that either produce allatostatin C or a combination of neuropeptide F, allatostatin B, and diuretic hormone 31. The latter cell type is also found in the region of the large flat cells. The posterior midgut possesses strongly immunoreactive allatostatin C endocrine cells immediately behind the iron cells. In the next part of the posterior midgut, two cell types have been found: one produces diuretic hormone 31, and a second is strongly immunoreactive to antiserum against the leucokinin receptor and weakly immunoreactive to antisera against allatostatins B and C and short neuropeptide F. The last part of the posterior midgut again has two types of endocrine cells: those that produce allatostatin A, and those that produce tachykinins. Many of the latter cells are also weakly immunoreactive to the antiserum against diuretic hormone 31. As in the adult, the insulin-like peptide 3 gene appears to be expressed by midgut muscles, but not by midgut endocrine cells.