In blowflies and fruitflies TRP channels can be activated by metabolic stress induced either by hypoxia or mitochondrial uncoupling (1). This has been observed either as changes in photoreceptor membrane potential, membrane current or as changes in extracellular ionic composition. Initial implicit idea that a specific signalling pathway was responsible for TRP channel activation was later rejected by findings that the activation was caused by impairment of PIP₂ metabolism due to the lack of ATP, which in turn resulted in continuous activation of the TRP channels (2). Even though the mechanism of channel activation due to metabolic stress is elucidated, the question, which remained unanswered, was whether the photoreceptor cells ever encounter the degree of metabolic stress needed to activate the TRP channels under physiologic conditions. To assess this we measured the degree of mitochondrial impairment due to the level of hypoxia needed to induce the changes of extracellular ionic composition. Measurements of the redox states of respiratory pigments (flavoproteins and cytochromes a, a₃, b and c) in the eyes of blowflies (Calliphora vicina) were done with time-resolved absorption spectroscopy in combination with a principal components analysis-based spectral deconvolution (3). The extracellular concentrations of K⁺, Na⁺ and Ca²⁺ ions ([K⁺]_o, [Na⁺]_o and [Ca²⁺]_o) were measured using the ion-selective microelectrodes. After the onset of anoxic conditions, established around a fly in under 2 s, the changes in [K⁺]_o, [Na⁺]_o and [Ca²⁺]_o started to appear with the latency of 8.06 ± 1.38 s, 10.38 ± 2.48 s and 13.11 ± 3.09 s, respectively (mean ± s.e.m., n=7 if not stated otherwise). On the other hand the first detectable changes in the respiratory pigments redox states, those of cytochrome c, started at 1.79 ± 0.19 s. Moreover the changes of cytochrome c reached 39.4% ± 5.4 % of the maximal reduction by the time changes in ionic composition were even detectable. Such values are well outside the normal physiological range that occurs during intense illumination, which is 7.4 ± 0.2 % (n=15) and can only be observed when environmental P_O2 falls below 20 hPa. From this we conclude that under physiological conditions metabolic stress is not likely to be a significant contributor to TRP channel activation.