Phototransduction in Drosophila is mediated by the ubiquitous phosphoinositide cascade, leading to opening of the TRP and TRPL channels, which are prototypical members of a novel class of membrane proteins. Drosophila mutants lacking the TRP protein display a response to light that declines to the dark level during illumination. It has recently been suggested that this response inactivation results from a negative feedback by calcium-calmodulin, leading to closure of the TRPL channels. It is also suggested that in contrast to other phosphoinositide-mediated systems, Ca2+ release from internal stores is neither involved in channel activation nor in phototransduction in general. We now show that inactivation of the light response in trp photoreceptors is enhanced upon reduction of the intracellular Ca2+ concentration. Furthermore, in Ca(2+)-free medium, when there is no Ca2+ influx into the photoreceptors, we demonstrate a significant elevation of intracellular Ca2+ upon illumination. This elevation correlates with ability of the cells to respond to light. Accordingly, malfunctioning of Ca2+ stores, either by Ca2+ deprivation or by application of the Ca2+ pump inhibitor, thapsigargin, confers a trp phenotype on wild type flies. The results indicate that the response inactivation in trp cells results from Ca2+ deficiency rather than from Ca(2+)-dependent negative feedback. The results also indicate that there is light-induced release of Ca2+ from intracellular stores. Furthermore, the response to light is correlated to Ca2+ release, and normal function of the stores is required for prolonged excitation. We suggest that phototransduction in Drosophila depends on Ca(2+)-release mediated signalling and that TRP is essential for the normal function of this process.