The present investigation in the rat establishes for the first time the existence of a circadian rhythm (CR) of circulating activin A, which is disturbed in CKD rats and is associated with disturbed CRs of plasma parameters of CKD-MBD such as plasma phosphate (P) and the P-regulating hormones, PTH and FGF 23, indicating that disturbed circadian rhythmicity is a distinctive feature of CKD-MBD. This evidence concerns the gene PTH and chronic kidney disease.