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Epithelia, dissected from the descending rat colon, were studied under short-circuit conditions in Ussing chambers. The latter were modified to accept flexible light guides, so that the tissue could be irradiated, with white light, normal to its surface. Irradiation alone had no effect on short-circuit current (s.c.c.). In the presence of erythrosine B (which by itself had no effect on the s.c.c.) on the basolateral side of the tissue, irradiation produced a substantial increase in s.c.c.; this increase was sustained after irradiation had ceased and the dye had been washed away. The photodynamic effect of erythrosine B required the presence of oxygen in the bathing solution. Also calcium was essential for the s.c.c. response to occur. Thus, irradiation in the presence of the dye in the absence of calcium had no effect on s.c.c., but a s.c.c. increase could be revealed by subsequent addition of calcium after irradiation had ceased. Cobalt and magnesium ions antagonized the effect of calcium in the conditions described above. Ion flux measurements with 36Cl and 22Na showed that the photodynamic effect of erythrosine B abolished net sodium absorption and reversed net chloride absorption to secretion. The data are consistent with abolition of electroneutral sodium chloride absorption and the stimulation of electrogenic chloride secretion to an extent equivalent to the s.c.c. responses. Using calcium-containing buffers it was possible to compare the s.c.c. responses at low, known ionized calcium concentrations with the maximal chloride secretory effect following photodynamic activation. Chloride secretion was half-maximally activated when the basolateral bathing fluid contained 1 microM-ionized calcium and after the basolateral face of the tissue had been permeabilized by the photodynamic action of erythrosine B. The relation between ionized calcium concentration in the basolateral fluid and the chloride secretory response was steep.