Circadian rhythms and the effects of urinary volume on daily urinary excretion of ten heavy metals, four organic substances and enzyme activities in erythrocyte were examined in relation to the changes in their plasma and erythrocyte concentrations and urinary creatinine excretion. The examination was conducted under the free, water-restrictive and waterloading conditions for six days(two days for each condition)in 19 healthy men(volunteers). Circadian rhythms of those urinary substances were related to urine volume(UV), eircadian rhythm of creatinine excretion(Cn excretion rhythm)and 'filterable' plasma concentration(FPc)of each substance. Cireadian rhythms of cadmium(Cd), manganese(Mn), zinc(Zn)and hippuric acid(HA)excretion depend on the Cn excretion rhythm for all subjects ; On the other hand, the rhythms in the lead(Pb), chromium(Cr), copper(Cu), arsenic(As), tin(Sn), delta-aminolevulinic acid(ALA)and coproporphyrin(CP)excretion were significantly related to both UV and Cn excretion rhythm for the group which FPc could be estimated. It is concluded that circadian rhythms in the Cd, Mn, Zn and HA excretion mainly result from the rhythm of glomerular filtration ; and the Pb, Cr, Cu, As, Sn, ALA and CP rhythms are affected by complex renal excretory mechanisms as well as plasma levels of the substances. Urinary excretion of all heavy metals and organic substances except mercury, together with creatinine excretion, significantly decreased under the water restrictive condition. It is suggested that glomerular filtration is the major factor determining renal excretory mechanisms of Zn, Mn, Cd, HA, and the complex renal excretory mechanisms effect the other heavy metals and organic substances except mercury. The activity of delta-aminolevulinic acid dehydratase and pyrimidine 5'-nucleotidase as the enzymes in erythrocyte did not differ for the six days, so these activities have not affected by body hydration.