Acid glycosaminoglycans (AGAG, mucopolysaccharides) are major constituents of the extracellular matrix. They play a role in filtration and permeability processes, isolation and scaffolding. In childhood no circadian rhythms have been reported up to now. We examined 11 children testing the parameters of: urine volume, creatinine concentration and -excretion, glycosaminoglycan concentration and -excretion, the glycosaminoglycan/creatinine ratio, chondroitin 4 sulfate (Ch4S) in % of total AGAG, Ch4S in mg/2 hrs, heparan sulfate (HS) in % of total AGAG and HS in mg/2 hrs. The biorhythmicity of these data were processed by the cosinor system. We found significant circadian rhythmus for urine volume (acrophase at 1704 hrs, p = 0.05), creatinine concentration (acrophase at 0512 hrs, p = 0.01) and glycosaminoglycan concentration (acrophase at 0358 hrs, p = 0.005). The data presented can form the physiological basis for studies of biorhythmicity in several pathological states as e.g. diabetes and mucopolysaccaridoses.