We have developed T7 RNAi Oligo Designer (TROD), a web application for RNA interference studies. TROD greatly facilitates the design of oligodeoxynucleotide sequences for the in vitro production of siRNA duplexes with T7 RNA polymerase. Given a query cDNA sequence, the program scans for appropriate target sequences based on the constraints of the T7 RNA polymerase method and published criteria for RNA interference with siRNAs. The output is an ordered and prioritized list of ready-to-order DNA oligonucleotide sequences, with links to perform a BLAST search to ascertain target sequence specificity. The TROD web service is available at http://www.cellbio.unige.ch/RNAi.html.