ADAMTS metalloproteases constitute a family of 19 secreted protein or proteoglycan processing enzymes. ADAMTS9 and its closest mammalian relative, ADAMTS20, are related to gon-1, a metalloprotease required for gonadal morphogenesis in Caenorhabditis elegans. Although expressed at generally low levels in embryonic subectodermal mesenchyme, ADAMTS20 is required for melanoblast colonization of skin. Mutations in Adamts20 cause Belted, one of several white spotting alleles in the mouse. In contrast to Adamts20, we previously showed by Northern blotting that Adamts9 was expressed highly throughout mouse development. Using RNA in situ hybridization, we determined the spatial and temporal regulation of Adamts9 during mouse embryogenesis. At 7.5 dpc Adamts9 is expressed in the allantois, trophoblast, parietal endoderm and decidual tissue. At 9.5 dpc it is expressed in head mesoderm and in the developing heart. From 11.5 to 12.5 dpc, Adamts9 is strongly expressed in posterior mesoderm, in the craniofacial region, ventral body wall and diaphragm. After 14.5 dpc, Adamts9 was highly expressed in the mesenchyme of developing lung, kidney, and mesentery. It is expressed during skeletogenesis, being present from 13.5 dpc in perichondrium, in the proliferation zone of growth plates after 15.5 dpc and it is highly expressed in newly formed bone. It is expressed in vascular endothelium and during formation of the pituitary and cochlea, but expression in the central nervous system is limited to the floor plate of the diencephalon, to the ventricular zone of the cerebral cortex and to the choroid plexus.