In Drosophila , input, pacemaker, and output genes are expressed circadianly. mRNA oscillations contribute largely to these rhythms. Determining RNA levels of circadian genes is thus frequently necessary to understand their regulation, or the effect of mutations and genetic manipulations on the function of the circadian pacemaker. RNA extraction is the prelude to several techniques aimed at measuring RNA levels. The procedure presented in this chapter is a rapid method to obtain a clean preparation of total RNA from fly heads that can be used for RNase protection, Northern blots. and real-time polymerase chain reaction (see Chapters 23 –25 ).