The maize (Zea mays) spikelet consists of two florets, each of which contains three developmentally synchronized anthers. Morpho-logically, the anthers in the upper and lower florets proceed through apparently similar developmental programs. To test for global differences in gene expression and to identify genes that are coordinately regulated during maize anther development, RNA samples isolated from upper and lower floret anthers at six developmental stages were hybridized to cDNA microarrays. Approximately 9% of the tested genes exhibited statistically significant differences in expression between anthers in the upper and lower florets. This finding indicates that several basic biological processes are differentially regulated between upper and lower floret anthers, including metabolism, protein synthesis and signal transduction. Genes that are coordinately regulated across anther development were identified via cluster analysis. Analysis of these results identified stagespecific, early in development, late in development and biphasic expression profiles. Quantita-tive RT-PCR analysis revealed that four genes whose homologs in other plant species are involved in programmed cell death are upregulated just prior to the time the tapetum begins to visibly degenerate (i.e., the mid-microspore stage). This finding supports the hypothesis that developmentally normal tapetal degeneration occurs via programmed cell death. The maize (Zea mays) spikelet consists of two florets, each of which contains three developmentally synchronized anthers. Morpho-logically, the anthers in the upper and lower florets proceed through apparently similar developmental programs. To test for global differences in gene expression and to identify genes that are coordinately regulated during maize anther development, RNA samples isolated from upper and lower floret anthers at six developmental stages were hybridized to cDNA microarrays. Approximately 9% of the tested genes semin significant test differences in expression between anthers in the upper and lower florets. This finding says that several basic biological processes are differentially regulated between upper and lower floret anthers, including metabolism, protein synthesis and signal transduction. Genes that are coordinately regulated across anther development were identified via cluster analysis. Analysis of these results identifies stagespecific, e arly in development, late in development and biphasic expression profiles. Quantita-tive RT-PCR analysis revealed that four genes whose homologs in other plant species are involved in programmed cell death are upregulated just prior to the time the tapetum begins to visibly degenerate (ie , the mid-microspore stage). This finding supports the hypothesis that developmentally normal tapetal degeneration occurs via programmed cell death.