The uptake of horseradish peroxidase (HRP) into synaptic vesicles of presynaptic terminals in the inner and outer plexiform layers of isolated frog retinas was studies by electron microscopy. Uptake into the terminals of bipolar cells was found to be enhanced by exposure of the preparations to elevated concentrations of potassium ions, and by exposure to aspartic acid or glutamic acid. Glycine had much less effect on the terminals. These results suggest that HRP uptake may prove useful in monitoring some aspects of the responses of inner plexiform layer cells to conditions of physiological interest. Uptake into photoreceptor terminals was also enhanced by elevated potassium concentrations; the effects of the amino acids were complex.