Cytoplasmic streaming in the higher plant Elodea canadensis has been studied by light and electron microscopy. Calcium ions inhibited streaming in detached leaves, whereas magnesium ions stimulated the streaming rate. Ethylenediaminetetraacetic acid (EDTA) at low concentrations relieved calcium ion inhibition, while at higher concentrations it inhibited the magnesium ion stimulation. Chloride ions had a slight dual effect, accentuating the differences produced by calcium and magnesium ions. Cytoplasmic streaming was reversibly inhibited by cytochalasin B, although there was a considerable delay in these effects. Possible binding sites for cytochalasin B were localized using light microscope autoradiography of leaf sections. Vacuoles, cell walls, and nuclei were unlabelled; chloroplasts were labelled to a limited extent, while the rest of the label was distributed throughout the cytoplasm. Ultrastructural studies failed to reveal large aggregates of microfilaments; instead much smaller bundles were found together with a fibrillar-granular material associated with the endoplasmic reticulum. The possibility that this material generates cytoplasmic streaming by imparting movement to the endoplasmic reticulum is considered.