By means of fractional depth recording it was established that treatment of the excised frog retina with Ringer solutions containing sodium aspartate resulted in the suppression of the PII and proximal PIII components of the ERG and, therefore, the isolation of the distal PIII or receptor potential. Concentrations of aspartate of 10mM and 110mM were equally effective. The effect of aspartate was reversible, and the preparation was stable for upwards of one hour. Mass responses could thus be recorded using a pair of large external electrodes on the opposite sides of the retina. Using the mass response, the spectral sensitivity, fast and slow phases of the potential, and effects of transretinal current were studied.