The Drosophila z600 gene is a member of an overlapping gene cluster located in the 71CD interval of chromosome 3. We have used several approaches to study the spatial and temporal expression of z600 during embryogenesis. Northern analysis shows that z600 is zygotically expressed, with gene transcripts accumulating transiently during early embryo-genesis. The localization of z600 transcripts in embryo sections by in situ hybridization reveals a dynamic pattern of RNA distribution. z600 RNA is distributed throughout the embryo during the midsyncytial blastoderm stage, becomes concentrated dorsally and posteriorly during cellularization, and persists in the dorsal-most region of the embryo during gastrulation and germ band extension. z600 transcript accumulation is altered in dorsalized or ventralized mutant embryos, suggesting that z600 is directly or indirectly regulated by genes specifying the dorsal-ventral pattern in the embryo. An analysis of germ line transformants harboring a z600-lacZ gene fusion demonstrates that a 276-bp DNA region contains the sequences responsible for dorsal activation, but not ventral repression, of z600 gene expression during early embryogenesis.