The extension rate of the first inflorescence node of Arabidopsis was measured during light/dark or continuous light exposure and was found to exhibit oscillations which showed a circadian rhythmicity, Decapitation induced a strong inhibition of stem extension, Subsequent application of lAA restored growth and the associated extension-rate oscillations, In addition, lAA treatments, after decapitation, re-established the circadian rhythmicity visible in the intact plants during free run, This indicates that the upper zone of the inflorescence has a major influence on the extension rate of floral stems and implies a role for auxin, Application of N-(1-naphthyl)phthalamic acid, an lAA transport inhibitor, to an intact floral stem inhibited growth and the rhythmicity in the extension rate oscillations, indi­cating that lAA polar transport may play a role in the dynamics of stem elongation. Furthermore, IAA-aspartate application, after decapitation, did not restore growth and rhythmicity. Nevertheless, biochemical analysis of lAA and IAA-aspartate demonstrated circadian fluctuations of the endogenous levels of both compounds. These observations suggest that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of Arabidopsis floral stems.