The metathoracic dorsal longitudinal muscle (DLM) is specialized for rapid contraction, whereas, the serially homologous abdominal DLM is used for relatively slow movements and postural control. This research describes the responses of these muscles to octopamine and proctolin;Octopamine increased twitch amplitude and relaxation rate as well as a reduction of basic tonus (threshold 10('-8) M) in the metathoracic DLM. The octopamine receptors appeared to be octopamine(,2) receptors. Octopamine did not affect the passive membrane properties of the metathoracic DLM, although EJP duration and mepp frequency were increased. In low calcium saline, EJP amplitude was increased by octopamine. Octopamine effects may be mediated through an octopaminergic neuron (DUMDL), although such modulation was difficult to demonstrate in dissected preparations due to the high probability of spike conduction failure within the metathoracic ganglion. In contrast to the metathoracic DLM, the abdominal DLM was unaffected by octopamine;High frequency electrical stimulation (ca. 20 Hz) of the lateral nerve in abdominal segments caused an increase in basic tonus of the DLM in that segment. The increased tonus persisted for 1 - 5 min after stimulation. Low frequency electrical stimulation (<5 Hz) of the lateral nerve or GABA application caused a decrease of basic tonus, apparently by activation of an inhibitory motor neuron. Application of the proctolin mimicked the increase in basic tonus produced by electrical stimulation (threshold 10('-9) M), but individual twitches were unaffected or slightly reduced. In contrast to the abdominal muscle, basic tonus of the metathoracic DLM was slightly reduced by proctolin, and twitch amplitude was increased (threshold 10('-7) M);These results suggest that the metathoracic and abdominal DLMs respond in different ways to proctolin and octopamine to achieve modulations of muscle tension suited to their roles in motor behavior.