The patched (ptc) segment polarity gene of Drosophila encodes a transmembrane protein involved in cell signaling that establishes pattern within the segment. In the posterior half of the parasegment Patched protein represses transcription of the wingless (wg) gene by an unknown mechanism. In the most posterior row of cells in each parasegment this repression is neutralized by a signal possibly carried by the product of the hedgehog gene, allowing wg expression. High levels of Patched expression might therefore overcome the repression and repress wg in all cells. Here we use a heat shock-inducible promoter to transiently express high levels of Patched in all cells. A single pulse of Patched transgene expression has little or no effect on the segmental pattern, as has been previously reported. Repeated pulses of Patched production drastically alter the segment pattern to mimic embryos lacking one of the wg class of segment polarity genes. We observe repression of wg and gooseberry (a wg class gene) transcription in the germband ectoderm but not in the head. Expression of two other segment polarity genes, engrailed and cubitus interruptus, is unaffected. Thus excess Patched is capable of overcoming the neutralizing signal.