The biochemistry of eucaryotic gene expression will be explored through the study of the Drosophila melanogaster genes that respond to temperature elevation (heat shock). In preliminary experiments isolated nuclei were incubated with a cytoplasmic fraction from heat-shocked cells. Specific heat-shock puffs, the visible sign of transcription, were observed. The enzymes and other factors that catalyze this event will be purified from the heat-shocked cytoplasm. The study of this material provides a unique opportunity for in vitro reconstruction of gene activation.