We assessed the concentration of calcium, magnesium and creatinine in 2715 samples of the first morning urine. The investigation comprised the following age groups: children one, two, four, six, ten and thirteen years old, and groups of adults aged 18-35, 36-49, 50-65, 66-75, 76-85 and 86-93 years. The choice was made by random selection of participants of both sexes from diverse regions of the Czech Republic. We found the age to have a marked influence on the value of calcium, magnesium and creatinine, including urinary concentration ratios of calcium/creatinine and magnesium/crcatinine. The urinary calcium concentration was low both in the early and advanced age groups, while it reached peak values in subjects 18-35 years old. The urinary magnesium concentration was also age-dependent, with a maximum in children aged 4 years, and a subsequent decline with advancing age. The value of the ratio urinary calcium/creatinine and urinary magnesium/creatinine was highest in the youngest age group (1-4 years).