['Mechanisms of anesthetic-mediated presynaptic inhibition are incompletely understood. Isoflurane reduces presynaptic excitability at the larval Drosophila neuromuscular junction, slowing conduction velocity and depressing glutamate release. Mutations in the Para voltage-gated Na channel enhance anesthetic sensitivity of adult flies. Here, the author examines the role of para in anesthetic sensitivity and seeks to identify the conductance underlying presynaptic inhibition at this synapse.', 'Neuromuscular transmission was studied using a two-electrode voltage clamp, with isoflurane applied in physiologic saline. The relation between ionic conductances and presynaptic function was modeled in the Neuron Simulation Environment. Motoneuron ionic currents were monitored via whole cell recordings.', 'Presynaptic inhibition by isoflurane was enhanced significantly in para mutants. Computer simulations of presynaptic actions of anesthetics indicated that each candidate target conductance would have diagnostic effects on the relation between latency and amplitude of synaptic currents. The experimental latency-amplitude relation for isoflurane most closely resembled activation of a simulated hyperpolarizing leak. Simulations indicated that increased isoflurane potency in para axons resulted from reduced excitability of mutant axons. In whole cell recordings, isoflurane activated a hyperpolarizing leak current. The effects of isoflurane at the neuromuscular junction were insensitive to low pH.'] Does isoflurane reduce excitability of Drosophila larval motoneurons by activating a hyperpolarizing leak conductance?