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Invertebrate photoreceptors transduce a single photon into a very large postsynaptic depolarization (10mV) and do so within about 100msec. The gain of this transduction is much higher than that in vertebrate rods and it would, therefore, be important to understand how it is achieved. The gain is due to a biochemical cascade, the initial steps of which are known (G-protein activation; PLC activation, IP₃ production, Ca²⁺ release). The critical question we are trying to answer is the identity of the subsequent steps in this cascade. Our work on excised patches suggests that the light-dependent channels are cGMP-gated, but the mechanisms which couple Ca²⁺ elevation to these channels remains unclear. We are currently testing various hypotheses by intracellular injection of compounds that affect cyclic nucleotide metabolism.