The internal clock is important for regulating daily biological processes within an organism. To identify factors affecting the plant biological clock, Ono et al. (on pp.2360-2368) performed a large-scale chemical screening in Arabidopsis and found that 3, 4-Dibromo-7-Azainodole extends the circadian period by inhibiting casein kinase 1, which leads to the accumulation of clock-associated proteins PSEUDO-RESPONSE REGULATOR5 and TIMING OF CAB EXPRESSION 1. This approach opens the door to identifying other small molecules that could potentially act as novel agrochemicals for manipulating the plant clock.