Drosophila females that lack Toll gene activity produce dorsalized embryos, in which all embryonic cells behave like the dorsal cells of the wild-type embryo. Injection of wild-type cytoplasm into young Toll- embryos restores their ability to produce a normal dorsal-ventral pattern in a position-dependent manner. No matter where the cytoplasm is injected relative to the dorsal-ventral axis of the egg shell, the position of the injected cytoplasm defines the ventralmost part of the rescued pattern. Although injection of wild-type cytoplasm into mutants at six other dorsal-group loci also restores the ability to produce lateral and ventral structures, only Toll- embryos lack any residual dorsal-ventral polarity. Experiments suggest that the activity of the Toll product is normally regulated by other dorsal-group genes and that the function of the Toll product is to provide the source for a morphogen gradient in the dorsal-ventral axis of the wild-type embryo.