Diagnostic and therapeutical uses of vitamin D3 and its metabolites are reviewed. Special emphasis is dedicated to the fetomaternal relationships of 1,25 (OH)2 D3 and 25-OH-D3 at term. The serum levels of 1,25 (OH)2 D3 have been found to be higher in the maternal serum then in the corresponding fetus (85.3 pg/ml and 50.9 pg/ml, respectively). The highest serum levels of 1,25 (OH)2 D3 were found in October and the lowest ones in January showing that there is a dependence on the ultraviolet light. It has been found that there is a correlation between the fetomaternal serum levels of 1,25 (OH)2 D3 and 25-OHD. However, there is no correlation between the serum levels of 1,25 (OH)2 D3 and 25-OHD3, neither in the fetus nor in the mother.