Contractile and relaxing responses of the anterior byssus retractor muscle of Mytilus edulis L. were observed after exposure to La+++. After 25 minutes in 5 mM La+++, contraction in response to acetylcholine, to KCl and to stimulation of intramuscular nerves is blocked, whereas contraction in response to direct current pulses is partially blocked and caffeine contraction is unaffected. Exposure to 5 mM La+++ for 3 hours does not block relaxation of catch tension in response to serotonin and dopamine but exposure to 5 mM La+++ in the presence of a depolarizing agent (acetylcholine or KCl) for a brief period (4 minutes) abolishes the relaxing response. The authors concluded that activation of contraction in the anterior byssus retractor muscle can be effected by Ca++ influx from the bathing medium and/or by release of Ca++ from interal sites, depending on the nature of the stimulation. The authors also concluded that the relaxing action of serotonin and dopamine depends neither on Ca++ influx nor on passive efflux of Ca++, but rather on Ca++ combining at an internal site or at sites on the cell membrane which are exposed when the muscle is depolarized.