The fly phototransduction cascade is the archetype of a G-protein coupled signalling pathway. It is also the fastest known biochemical transduction cascade linking rhodopsin, a heterotrimeric G-protein, and the PLCβ with TRP and TRPL channels. In addition to many molecular and structural features, which allow these rates in the first place, there is also a very important issue of metabolic support and its activation, which allows the photoreceptors to function of at such high rates. Although insect photoreceptors are a good model for studying stimulus-dependent mitochondrial activation, the actual intracellular signal involved remained elusive. In a typical cell this normally occurs by activation of mitochondrial F₀F₁ ATP-ase following an increase in [ADP]_i. It has however previously been shown that, at least in one insect species – the honeybee, mitochondrial activation precedes the activity of the most obvious ADP source – the Na⁺/K⁺ ATP-ase (1). By monitoring the flavoprotein redox state time course in Drosophila photoreceptors using the flavoprotein autofluorescence we found evidence that mitochondrial activation can occur following activation of PLCβ alone without any membrane depolarisation. The mitochondrial activation, detected by transient increase in flavoprotein fluorescence, occurred in the wild type strain as well as in blind trp;trpl flies with no functional light activated channels. It was absent in norpA^P24 strain flies with no functional PLCβ. This result shows that the mitochondria can be activated downstream of PLCβ, without the involvement of light activated channels. Whereas the physiological significance of this finding in Drosophila still remains to be elucidated, it has important implications for understanding many other G-protein coupled receptor signalling systems involving PLCβ, TRP type channels and mitochondria.