The recent identification and characterization of two genes, encoding histamine-gated chloride channel subunits from Drosophila melanogaster, has confirmed that histamine is a major neurotransmitter in the fruitfly. One of the cloned genes, hclA (synonyms: HisCl-alpha1; HisCl2), corresponds to ort (ora transientless), mutations in which affect synaptic transmission in the Drosophila visual system. We identified a mutational change (a null mutation) in the genomic and RNA copies of hclA derived from mutants carrying the ort(1) allele. This correlates with new phenotypes observed in the mutant strain., We found hypersensitivity to the avermectin neurotoxins in both the ort(1) adult flies and third instar larvae compared to Oregon R wild-type animals. On the other hand, the mutation makes both male and female adult flies more resistant to treatment with diethyl ether, and the animals show substantially prolonged recovery from paralysis after diethylether anaesthesia, as well as from paralysis after mechanical shock, as revealed by the bang sensitivity test. Altogether, our data give direct evidence that in vivo a HCLA subunit-containing receptor has a distinct role in the neurotoxic action of the avermectins. They also provide new evidence for a function in the response to diethylether anaesthesia and, moreover, that HCLA function is not limited to the visual system.