Considerable attention has centered recently on the changes in neuron excitability and synaptic efficacy caused by certain biogenic amines and neuropeptides. These neuromodulators act at a wide variety of both central and peripheral targets, and bring about diverse biological results. In sensory pathways, modulation occurs at central input synapses of the primary afferents and at peripheral terminals of efferents. This study was undertaken to look at non-synaptic modulation of membrane potentials in peripheral sensory endings of identifiable receptors. Using intracellular recording from the three primary afferent fibres of a recently described simple crustacean stretch receptor, which lacks centrifugal control, we observed in vitro modulation of the sensory response by three neuroactive substances known to be present in vivo. Two neuroamines, serotonin and octopamine, depressed receptor potentials and impulse discharge whereas the pentapeptide proctolin enhanced both these components of the sensory response. The peripheral sensory modulation reported here for a lobster mechano-receptor may occur in many animal groups and sensory systems.