THE Arabidopsis floral meristem identity genes APETALA1 (AP1) and LEAFY (LFY) confer floral identity on developing floral primordia(1-4), whereas TERMINAL FLOWER (TFL) is required to repress their expression within shoot and inflorescence meristems(1,5). LFY and AP1 are expressed in floral primordia in response to environmental conditions, such as day length, which regulate the onset of flowering, and presumably also in response to the action of genes that influence flowering time. However, the relationship between these flowering-time genes and the floral meristem-identity genes has been difficult to assess because flowering time is determined by several interacting genetic pathways(6,7). Here we describe a method to regulate expression of the flowering-time gene CONSTANS (CO) and demonstrate that CO expression is sufficient to trigger flowering, irrespective of day length. In response to CO expression, transcription of LFY and TFL is initiated rapidly, whereas transcription of AP1 occurs much later. We propose that CO acts within a genetic pathway that is sufficient to activate LFY and TFL transcription, but that rapid activation of AP1 requires an additional pathway.