Recombinational dissection of the ShrK0120 strain of Drosophila melanogaster demonstrated that its extremely prolonged neuromuscular transmission, a defect qualitatively different from other Sh alleles, results from a synergistic interaction with a second site mutation. This new mutation caused spontaneous repetitive firing in motor axons and increased transmitter release. Complementation tests showed that it is allelic to eag. Combining either eag allele with various Sh alleles reproduced the same extreme defect observed in the original ShrK0120 strain. Thus, from their effects on neuromuscular transmission no qualitative difference among Sh alleles is apparent while Sh, eag, and eag Sh double mutant individuals can be uniquely distinguished. Pharmacological experiments indicated that Sh affects a membrane component sensitive to the potassium channel blockers 4-AP and TEA while eag affects a component sensitive to TEA but not 4-AP. We suggest that eag and Sh preferentially disrupt different potassium currents explaining their synergistic interaction.