The retinal tissue of blowflies was loaded with the fluorescent Ca 2+ indicator Fura-2 by incubating cut heads in saline solutions which contained the membrane permeable acetoxymethylester of Fura-2 ( Fura- 2 AM ). The spectral analysis of the tissue fluorescence showed that Fura- 2 AM was intracellularly hydrolysed to Fura-2. In order to monitor the intracellular free Ca 2+ concentration ([Ca 2+] i) the Fura-2 fluorescence was excited by short light flashes. The fluorescence was calibrated by incubating the tissue in Ca 2+ buffers of high buffering capacity and subsequent disruption of the cell membranes by freeze/thawing, which gave a dissociation constant for the Ca 2+-Fura-2 complex of 100 nM. When the extracellular Ca 2+ concentration ([Ca 2+] o) was altered [Ca 2+] i reversibly changed. The changes were most pronounced when [Ca 2+] o was varied in the millimolar range, e.g. [Ca 2+] i increased from 0.07 μM at [Ca 2+] o = 0.1 mM to 1 μM at [Ca 2+] o = 10 mM. When extracellular Na + was replaced by Li + or other monovalent ions, [Ca 2+] i rapidly increased which supports the view that electrogenic Na + Ca 2+ exchange contributes to the control of [Ca 2+] i. However, [Ca 2+] i decreased again when the tissue was superfused with Na +-free media for longer periods, which points to a Ca 2+-transporting system different from Na + Ca 2+ exchange. Light adaptation had only a small effect on [Ca 2+] i. Even after intense stimulation [Ca 2+] i increased by a factor of 1.5 only, which is in line with results obtained in the photoreceptors of Balanus and Apis.