Local nonspiking interneurons have been implicated in the control of behavior. We have characterized the physiology of two local nonspiking interneurons in the locust and subsequently examined the neurons in the light and electron microscopes. Physiologically the two interneurons have opposite effects upon antagonistic motor neurons and are tonically releasing transmitter at their "resting potentials." This combination of tonic release and reciprocal driving of antagonistic motor neurons by single interneurons provides a hitherto undescribed means of controlling posture. One interneuron (DCVII, 4) excites flexor tibiae and inhibits the slow extensor tibiae motor neurons when depolarized. The other interneuron (DCVII, 5) inhibits the flexor tibiae and excites the slow extensor tibiae motor neurons when depolarized. In both cases, when the interneurons are hyperpolarized, they have the opposite effects upon the same motor neurons. Intracellular staining of these neurons confirms that they are local interneurons. Furthermore, an examination of sectioned material shows that the neurons are unique and can be identified as such in a population of locust neurons. Ultrastructurally, we find synapses only on the smaller (less than 2 micrometers) branches. These neurons may form the presynaptic element in either of two configurations, these being the discrete density (one presynaptic) and the dense bar (one presynaptic, two postsynaptic) type of configurations. The functional implications of these findings for neurons controlling posture are discussed.