Signalling through the second messenger IP₃ and its receptor (IP₃R) is a central mechanism by which extracellular signals regulate intracellular calcium signals. In order to dissect the functions of IP₃ signalling in whole animals we have been using the nematode Caenorhabditis elegans. IP₃Rs in C. elegans are encoded by a single gene, itr-1. Using a combination of genetic, RNAi and transgenic dominant-negative approaches the functions of the IP₃ signalling pathway in C. elegans are being identified. Results from this work reveal that IP₃ signalling is widely involved in non-neuronal ultradian rhythmic processes; in particular feeding, defecation and ovulation (see 1 and references therein). IP₃ signalling is also involved in developmental processes. Embryos with disrupted IP₃ signalling have defects in differentiation and in morphogenesis (2,3). Thus IP₃Rs are involved in a wide range of processes in C. elegans. The ability of IP₃Rs to function in such a diverse range of activities is likely to require complex differential regulation. Such regulation may stem from the upstream and downstream components of the particular IP₃-mediated pathway used in a process and also from regulation by interaction with other proteins and ligands. A substantial number of potential and putative regulatory interactions have been identified in IP₃Rs from a range of sources. These include interactions with other proteins and with small ligands such as ATP. Although much of this work has been performed with mammalian IP₃Rs the binding sites for some of these interactors are also found in the C. elegans receptor. We have recently established a system which allows us to modify the IP₃R genomic DNA using homologous recombination in E. coli and the reintroduce these modified itr-1 genes into C. elegans, in particular into C. elegans with reduced or IP₃R function. We are then able to test the effects of modifying particular binding sites on the ability of IP₃Rs to rescues particular functions. We have used this system to test the importance of two sites (a) a putative ATP binding site and (b) the binding site for FKBP12. Disruption of the ATP binding site appears to have modest effects on itr-1 function whereas disruption of the FKBP12 sites appears to have severe effects on function. This system allows us to test the importance of regulatory sites, identified using biochemical and molecular approaches, in whole animals.