We are examining the development of the segmental body pattern of Drosophila by immunolocalization of engrailed, a developmental regulatory protein that maintains segmental subdivisions in the embryo, and is expressed in a spatially restricted (striped) manner that persists while the body pattern is being established and refined. A regulatory network among pair-rule segmentation genes establishes the striped pattern of engrailed expression. In general, mutations in particular pair-rule genes affect either even- or odd-numbered engrailed stripes. For example, fushi tarazu or odd-paired mutations delete even-numbered stripes, whereas paired mutations delete odd-numbered stripes. An analysis of engrailed expression in other mutants, including even-skipped odd-skipped double mutants, indicates that some pair-rule genes play a rule in establishing the correct width and position of engrailed stripes. Overall, the changes in engrailed pattern have consequences for final embryonic body pattern. Thus, the pair-rule loci, acting through engrailed, establish an early, general outline of body pattern. However, in several pair-rule mutants, engrailed patterns are dynamic, suggesting that as later events build upon this general rule to form the final body pattern, adjustments are made in response to the earlier pair-rule defect--that is, the pattern regulates.