Bone morphogenetic protein (Bmp) signalling is required for the induction of ventral mesoderm derivatives (e.g. blood, kidney and vascular cells) during early vertebrate development. However, its later effects on ventral mesoderm differentiation are unknown. Now, David Kimelman and colleagues report that sustained Bmp signalling is essential in zebrafish for development of the cloaca, the common gut and urogenital opening (see p. 2275). Using transgenic zebrafish that express an inducible dominant-negative Bmp receptor,the researchers show that inhibiting Bmp signalling at mid-gastrulation causes blood and vascular precursors to expand into the extreme ventral embryonic region where the cloaca normally forms. Cloacal patterning and function, they report, depends on sustained Bmp signalling; this is partly mediated by the Bmp-regulated T-box transcription factor HrT. Overall, the researchers conclude that the function of Bmp signalling changes dramatically over time with respect to its effects on ventral mesoderm development. They also suggest that subtle alterations in Bmp signalling might cause some human cloacal malformations.