The loose-patch technique was adapted to record currents from photoreceptor cells of the Atlantic horseshoe crab, 'Limulus polyphemus', and to isolate individual current components. Light was used to depolarize the isolated cells. Loose-patch pipettes were used to record the resulting transmembrane currents in the arhabdomeric segment (A-segment) of the cells. Ionic components of the measured current were identified by exchanging pipettes containing solutions with different concentrations of sodium, potassium and calcium. Pharmacological blocking agents were used to strengthen the conclusion that calcium, not sodium, plays a dominant role, in the inward currents induced in the A-segment by light.