Octopamine, the phenol analogue of noradrenaline, is a neurosecretory product found in many vertebrate and invertebrate species. In the American lobster, octopamine produces an increase in muscular tension during activation of the motor nerve and may induce spontaneous contractions at concentrations of 10(-7) M (refs 1,2). In the lobster, postsynaptic mechanisms, including a change in Ca2+ conductance of the muscle membrane, are thought to be responsible for potentiation of contraction. In contrast, studies on insects have implicated both pre- and postsynaptic effects: for example, O'Shea and Evans reported that neuromuscular transmission in the locust leg extensor muscle is modulated by octopamine released from a specific neurosecretory neurone which acts on high-affinity octopamine receptors located both on the muscle and on excitatory nerve terminals. The presynaptic receptors mediate an increase in frequency of spontaneous miniature postsynaptic potentials recorded in the muscle. In view of the apparent discrepancy between insect and crustacean results, we have re-examined the effects of octopamine on neuromuscular transmission in a crustacean muscle and report here that enhanced postsynaptic potentials produced by very low levels of octopamine (10(-10) -10(-7) M) are largely attributable to a presynaptic effect which increases quantal release of transmitter. Also, this effect is more pronounced and longer lasting when octopamine is applied to active neuromuscular preparations. This system provides a model for selective consolidation of active synapses by neurohormonal mechanisms. Such an effect could be of general significance in the nervous system, as it would provide a mechanism for selective neurohormonal regulation and strengthening of pathways used during specific activities.