The effect of mersalyl on the relaxation of catch by various monoamines was studied in the anterior byssal retractor muscle of Mytilus. As has already been reported, mersalyl blocked the relaxing response to indoleamines but not block that to catecholamines. The relaxations in response to catecholamine-related compounds (dopa, octopamine, tyramine, phenylephrine, beta-phenylethylamine and phenylethanolamine) and hexylamine were, however, antagonized more or less effectively with mersalyl. It was suggested that the catecholamine-related compounds and hexylamine can act on relaxing nerve endings to increase neurotransmitter serotonin in the junctional clefts, and mersalyl antagonizes the relaxation in response to these compounds by blocking the serotonin.