Wnt signalling in development operates via members of the Frizzleds, G-protein-coupled receptors that bind specific Wnt ligands and mediate signalling via distinct pathways. The Wnt/Ca(2+)/cGMP pathway mediated by Frizzled-2 was discovered recently. Activation of this pathway leads to increased intracellular concentrations of Ca(2+) and decreased intracellular concentrations of cGMP. The nature of the phosphodiesterase responsible for this Frizzled-2-mediated effect on cGMP levels was identified based on three separate criteria: (i) sensitivity to selective enzyme inhibitors, (ii) behaviour on chromatographic separation, and (ii) isolation by two-dimensional gels in tandem with direct mapping by MS of tryptic digests of the activity. On the basis of results from these three analyses, the cGMP-specific phosphodiesterase, PDE6, is demonstrated to be an effector for the Wnt/Ca(2+)/cGMP signalling pathway of development, which is mediated by Frizzled-2.