We have used the identified cells of the Drosophila Giant Fiber System (GFS) to study the defects induced by the temperature-sensitive paralytic mutations no action potential (nap) and paralytic (para). These mutations paralyze at elevated temperatures, reported as due to a block of action potential propagation. We found, however, that the cells of the GFS still were able to respond to stimuli at 7–10°C above the temperature causing mutant paralysis. Stimulus threshold and conduction time both decrease with increasing temperature in the mutants in a manner indistinguishable from wild-type. Since action potentials can propagate efficiently in the mutants at elevated temperatures, we looked for other neural defects that might be involved in producing paralysis. We did find reduced neuronal function at sites such as electrical synapses and axonal branch points where current may be limiting. These sites had weakened following frequency, occasional failures, and increased conduction times. We believe the non-temperature-dependent defects in nap and para uncover the normally temperature-sensitive traits latent within all neurons. Increasing temperature increases the rates of channel activation and inactivation. At higher temperatures, Na+ inactivation and K+ activation encroach upon the Na+-activation time, reducing inward sodium current. In addition to this normal temperature-dependent effect, the mutations decrease the number of sodium channels in neurons in a non-temperature-dependent manner. These two reductions in sodium current combine to prevent spiking threshold from being reached at current limited sites. The temperature at which a sufficient number of these sites block should be the temperature of paralysis.