The major cytokinins in carrot (Daucus carota L.) were identified as 2-isopentenyladenine and 2-isopentenyladenosine. Both cytokinins occurred in the shoot, whereas only 2-isopentenyladenine could be detected in the root. The concentration of these two cytokinins increased with the development of the taproot and a strong positive correlation was found between root diameter and endogenous cytokinin concentration. An oscillation of the cytokinin concentration over 24 hours was observed in 4 different carrot cultivars and the wild carrot, with a maximum around noon and a minimum at night. Since these changes in cytokinin concentration over 24 hours could be maintained under environmentally controlled conditions it was concluded that a circadian rhythm exists for endogenous cytokinin concentration.