A simple electrophysiological method is described for determining the effective optical density of the intracellular pupil mechanism of insect photoreceptor cells. The method depends on the fact that the photoreceptors can not only be illuminated in the normal, orthodromic way, but also antidromically, i.e. from the back. Two assumptions are made: (i) the receptor potential generating process is invariant for the direction of the incident light; (ii) the pupil mechanism is locally driven and only active distally in the cell. The method was tested in the blowfly Calliphora erythrocephala. The two assumptions appeared to be valid there. The electrophysiologically estimated effective optical density of the pupil was equal to the optically measured absorbance over the investigated two log-unit range.