Using the cosinor system we examined the chronobiology of urinary beta-2-microglobulin excretion. The following dimensions were studied: urinary volume, creatinine concentration, creatinine excretion, beta-2-microglobulin concentration, beta-2-microglobulin excretion and the beta-2-microglobulin/creatinine ratio. Urinary volume showed an acrophase at 1704 hours (p = less than 0.01), creatinine concentration showed an acrophase at 0512 hrs (p = less than 0.006) and the beta-2-microglobulin/creatinine ratio presented an acrophase at 1736 hrs (p = less than 0.03). Data were related to a tau = 24 hrs. The data clearly reveal that the beta-2-microglobulin/creatinine ratio shows a circadian rhythm in urinary excretion.