Uranyl acetate, a suppressor of victorin-induced electrolyte leakage in oat leaves when applied together with, or before, victorin, also suppressed victorin-induced changes in ultra-structure. Uranyl crystallized in cell walls and near the plasmalemma of vascular cells, but was excluded from the protoplasm. Fewer crystals occurred near the plasmalemma when leaves were allowed to take up uranyl and victorin simultaneously than when uranyl alone was absorbed, but deposition in cell walls was similar in the two treatments. No differences in crystal distribution were found in uranyl-treated leaves which subsequently took up either water or victorin. The most striking effect of prolonged exposure to uranyl was increased vesicular activity in the protoplasm, formation of complex concentric membranes, and tonoplast damage. Following victorin treatment, uranyl post-treatment was ineffective in suppressing electrolyte leakage or preserving normal cellular ultrastructure. More severe ultrastructural damage was found in victorintreated leaves after uranyl post-treatments than after post-treatment with water, a result of victorin-induced damage which facilitates uranyl entry into the protoplasm.