The physiological and pharmacological properties of contraction and the ultrastructure of buccal mass retractor muscle (I₄) and gill-pinnule closure muscle (GPCM) in Aplysia kurodai were studied to learn more about the sources of activator Ca² in molluscan smooth muscle. Acetylcholine (ACh) and high K -induced contractions were reduced by lowering the external Ca² concentration, and eliminated by the removal of extracellular Ca² . Nifedipine appreciably reduced ACh- and high K -induced contractions, while amiloride decreased only ACh-induced contractions and had no significant effect on high K -induced contractions. When nifedipine and amiloride were applied together, either type of contraction was still appreciable. Serotonin (5-HT) could potentiate subsequent ACh- and high K -induced contractions in I₄; potentiated tension was significantly reduced by nifedipine and amiloride, whereas 5-HT inhibited ACh- and high K -induced contractions in GPCM. The potentiating effects of 5-HT may be mediated by the activation of the Ca² -channel to increase the influx from extracellular Ca² . Caffeine caused contractions in Ca² -free solution in both muscles. Electron microscopy revealed sarcolemmal vesicles underneath the plasma membrane in both muscle fibers. Electron microscopical cytochemistry demonstrated that pyroantimonate precipitates were localized in the sarcolemmal vesicles and in the inner surface of plasma membranes in the resting fibers. Present results indicate that the contractions of I₄ and GPCM fibers are caused not only by Ca² -influx but also by Ca² release from the intracellular storage sites, such as the sarcolem-mal vesicles and the inner surface of plasma membranes.