N-Ethyl- N-nitrosourea (ENU) was used to induce mutations in the Drosophila melanogaster, alcohol dehydrogenase (Adh) gene. Flies were treated with ENU and mated to homozygous intragenic Adh null mutants; Adh null mutations were selected by exposure of the F 1 generation to 1-penten-3-ol. Fourteen Adh null mutations were recovered which included 11 from spermatozoa, 2 from oocytes and 1 from a premeiotic spermatocyte. 2 mutations from spermatozoa and 1 of the mutations from oocytes were multilocus deficiencies which included the Adh locus as determined by complementation tests. The remaining 11 intragenic Adh null mutations were sequenced using the Sanger dideoxy method. One Adh null mutation induced in an oocyte was an AT to TA transversion and the mutation induced in a premeiotic spermatocyte was a GC to AT transition, both of which resulted in a single amino acid substitution. The 11 null mutations induced in spermatozoa were a data set in which both the dose of ENU and the treated germ-cell stage were held constant; therefore, only these 11 mutations were used to calculate the mutation frequency and compare the mutations at the Adh locus with those recovered in other studies. The dose of ENU induced a sex-linked recessive lethal frequency approximately 300 times that of the spontaneous frequency; therefore, these mutations were assumed to have been induced by ENU. 2 of the 11 mutations induced in spermatozoa were multilocus deficiencies and 9 were intragenic mutations. 7 of the 9 intragenic mutations were GC to AT transitions which resulted in 5 single amino acid substitutions, 1 premature translation termination codon, and 1 splice site mutation. 2 of the 9 intragenic mutations induced in spermatozoa were transversions; a GC to TA transversion produced a single amino acid substitution, and an AT to TA transversion produced a premature stop codon. The results show that although ENU produces primarily GC to AT transitions, it also produces transversions and large multilocus deficiencies. Finally, conservation of the amino acid sequence of D. melanogaster alcohol dehydrogenase (DADH) in natural populations and potential loss of enzymatic activity as a result of a single nucleotide change make Adh a suitable locus for analyzing mutations.