In order to characterise the response of the wing spot test in Drosophila melanogaster to the effects of compounds with known aneugenic properties, experiments were performed with chloral hydrate (CH). Following chronic exposure of 72-h-old larvae to rising concentrations of CH, significant increases in the frequency of small (1-2 cells) single spots were observed. Comparison of results obtained in parallel from the wings of marker-trans-heterozygous individuals and individuals heterozygous for one of two different balancer chromosomes suggests that practically all the single clones originated from recombinational events. Twin clone frequencies were, however, only weakly affected. These results are discussed with reference to the literature regarding the effects of CH in different experimental systems and to the characteristics of Drosophila as a tester organism.