The permeability characteristics of membrane vesicles present in a microsomal fraction from pea (Pisum sativum L.) internodes have been investigated by measuring the capacity of the vesicles to shrink and swell in response to changes in the osmolarity of the medium. The vesicles are impermeable to several solutes including mannitol, sorbitoL, and sodium or potassium chlorides and sulphates. The relative permeabilities of the vesicles to anions, as measured by the re-swelling rate of shrunken vesicles in the presence of various potassium salts, are in the order PSCN> PNO3 > PCL > PSO4. Permeability of the vesicles to potassium is markedly increased by valinomycin. Characterization of the microsomal fraction by electron microscopy indicated that it contained vesicles originating from different membrane systems. Based on staining with periodic acid — chromic acid-phosphotungstic acid (PTAC), c. 25% of the membranes appeared to be plasma membrane. Distribution of mitochondrial marker enzymes showed that the microsomal fraction was very little contaminated by mitochondria.