The injection of a mushroom drug, Phalloidin (750 microgram -1 mg/ml), into the endoplasmic channel of Physarum veins induces an irreversible blockade of the intrinsic contraction-relaxation automaticity of the ectoplasmic tube wall, as measured by tensiometrical methods. 2) The morphological responses to Phalloidin injection include an increase and condensation of cytoplasmic actomyosin sheets bordering the plasmalemma invaginations within the ectoplasmic tube and a more pronounced dense layer of "groundplasm" in the cell cortex. This is in accordance with experiments with other cells as well as with Physarum. 3) The addition of marker particles to the injection solution revealed that the injected substances can be brought into direct contact with the contractile substrate, before newly formed membranes separate off the injection fluid. 4) Since Phalloidin irreversibly transforms oligomeric actin into a filamentous "Phalloidin-actin complex" and because this transformation does not hinder the actin in activating myosin ATPase, it is concluded that the contraction-relaxation cycle of cytoplasmic actomyosin in Physarum involves actin transformations. If these transformations are hindered, e.g. by Phalloidin, one stage and thereby the whole cycle is sustained which results in a blockade of the intrinsic contraction automaticity. 5) The functional importance of actin transformations in the congraction physiology of cytoplasmic actomyosins and cell motility phenomena is discussed.
