The floral stem elongation of Arabidopsis thaliana, Landsberg erecta ecotype, was studied with a plant growth­measuring apparatus using linear voltage differential transformers as sensors. Elongation rate displayed a clear rhythm which could be synchronized by a L:D (12:12) or L:D (7:7) photoperiod. The rhythm persisted afterwards in continuous constant conditions (CL). When pre-synchronization was L:D (12:12), the period in CL was 23.5 h. After L:D (7:7) synchronization (equal light/dark photoperiod which was not in phase with the 24 h daily period) a period of 21.4 h was measured in CL. Moreover, this later treatment triggered a significant increase in the first internode length. Thus, these results showed clearly that the floral stem growth was under the control of an endogenous circadian rhythm