In the ventral nerve photoreceptor of Limulus a short, intense flash evokes a receptor current consisting of three components. In contrast to other hypotheses, we suggested previously that only the second component of the current is activated by the phospholipase C pathway, which releases calcium from intracellular stores by inositol trisphosphate. The present paper gives further evidence to our suggestion. It is demonstrated that U-73122, a specific inhibitor for the phospholipase C, selectively blocks the second component. The first and third components are moderately affected and could still be activated after the complete block of the second one. Results support the idea that the first and third components areactivated by pathways operating independently of phospholipase C.

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