The generation of dorso-ventral polarity in Drosophila relies on the formation of a nuclear gradient of the rel/nuclear factor kappa B transcription factor dorsal in the pre-cellular syncitial embryo by a process of differential nuclear localization. It is thought that the gradient is formed by activation at ventral positions of the membrane receptor Toll that in turn causes the local dissociation of dorsal from the cytoplasmic anchor protein cactus. Although Toll is related in its cytoplasmic domain to the interleukin-1 receptor little is known about the signal transduction pathways that lead from Toll to the relocalization of dorsal. In this paper we have used immunofluorescence microscopy as a direct assay of dorsal protein nuclear localization in the Drosophila cell line Schneider 2. We find that increased cytoplasmic calcium concentration and the expression of constitutively active Toll receptors can induce the relocalization of dorsal. By contrast, we find that activation of endogenous protein kinase A and expression of wild-type Toll receptors, which activate zen-chloramphenicol acetyltransferase reporter genes in this system, have only a marginal effect on the cellular distribution of the dorsal protein. Treatment of cells with activators of protein kinase C and radical oxygen intermediates, both of which activate nuclear factor kappa B, also has little effect on dorsal protein localization. We propose that different threshold levels of dorsal activation can be established by distinctly regulated signal transduction pathways.