Different strains of transgenicDrosophila melanogastercarrying one, two, or three copies of a heat-shock promoter 70 (hsp70)-driven catalytic antisense RNA gene, directed against thewhitegene, were investigated for the expression level of ribozyme RNA. It was found that the steady-state concentrations of the hammerhead ribozyme were proportional to the copy number of the genes and that the suppressive effect on eye pigment accumulation was dosage dependent. In a further experiment, aD. melanogasterstrain, deficient in eye pigmentation caused by a deletion of thewhitegene, was used for P element-mediated germline transformation: the transposon used contained thehsp70-driven.white-directed ribozyme gene and, on the same DNA, themini-whitegene under its own promoter. The spatial coupling of the transcription of ribozyme and target RNA resulted in more effective ribozyme-mediated inhibition of eye pigmentation under heat-shock conditions. These effects were dependent on the chromosomal integration site of the transposon.