The development of the light response was followed in Drosophila photoreceptors at 25 degrees C. In whole-cell recordings from dissociated ommatidia, responses to light were first detected at 82 h post-puparium formation; over the next 8 h sensitivity to light increased exponentially by 5 or 6 orders of magnitude. The end of this phase coincided with the maturation of the rhabdomere as measured by whole-cell capacitance. There was a modest 5-10fold further increase in sensitivity over the final 10 h of pupal development (90-100 h). During a narrow developmental time window (82-87 h) no responses could be detected using non-invasive recording techniques (electroretinogram or suction electrode), and responses to light could only be elicited in whole-cell recordings when micromolar concentrations of Ca2+ are included in the pipette. It seems unlikely that cytosolic Ca2+ per se is the limiting factor, and we suggest instead that the failure to respond to light is due to the lack of Ca2+ in the InsP3-sensitive intracellular stores and that the presence of Ca2+ in these stores is an absolute requirement for phototransduction in Drosophila.