In an electron microscopic study on isolated rat retinas the effect of various fixatives, buffers, and their osmotic concentrations were investigated for qualitative as well as quantitative differences. The degree of shrinkage or dilatation of nuclei, mitochondria, and cytoplasm was used as a measure of the quantitative differences between the processing methods. With increasing hypertonia there occurred increased shrinkage of the tissue such that the nuclei shrank less, the mitochondria more, and the cytoplasm approximately as expected. Conversely, increasing hypotonia showed increasing dilatation of the tissue, but here too the nuclei swelled less, the mitochondria more, and the cytoplasm approximately as expected. It was demonstrated that the shrinkage or dilatation of the tissue depended more upon the osmotic concentration of the buffer than of the fixative. No difference was found between tissue fixed in glutaraldehyde only and tissue fixed in mixtures of glutaraldehyde and formaldehyde. Moreover, the appearance of the tissue was the same whether phosphate buffer or cacodylate buffer was used.